625377009A
Controlling Mlutfon
from the Manufacturing
and Coating
of Metal Roducts
Metal Coating Air Pollution Control
Do not WEED. This document
should be retained in the EPA
Region 5 Library Collection.
EPA Technology Transfer Seminar Publication
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EPA-625/3-77-009
CONTROLLING POLLUTION
FROM THE MANUFACTURING
& COATING OF METAL PRODUCTS
METAL COATING
AIR POLLUTION CONTROL-I
U.S. ENVIRONMENTAL PROTECTION AGENCY
Environmental Research Information Center • Technology Transfer
U.S. Eiwiiaiimwtal PmUctlon Agwcy
Region 5, Library (PL-12J)
l««^IWII •»» MM/ t> «-•••*'
7? West Jackson Bpulcvird, 12th Ftoqr
Chicago, »L 60604-3590
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ACKNOWLEDGEMENTS
This seminar publication contains materials prepared for the U.S. Envi-
ronmental Protection Agency Technology Transfer Program and has been
presented at Technology Transfer design seminars throughout the United
States.
The technical information in this publication was prepared by Burton J.
Sutker of Foster D. Snell, Inc., and Uday Potankar of JACA Corporation.
NOTICE
The mention of trade names or commercial products in this publication
is for illustration purposes, and does not constitute endorsement or recom-
mendation for use by the U.S. Environmental Protection Agency.
LJ '"I F"rr-'!rrv^vsA~* * -p ~- j ..
tn^c^,....,, , .-- Agency
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INTRODUCTION
Recent findings of high levels of hydrocarbons in the nation's air have spurred
renewed activity by the U.S. Environmental Protection Agency (EPA) toward their
control. One result will be more emphasis on reducing hydrocarbon emissions from
industrial activity. EPA's Office of Air Quality Planning and Standards will shortly
publish guidelines to the states for control of these emissions in several industries,
including metal coating. In the next 2 years, federal standards for new plant construc-
tion in these industries are also expected.
In the metal painting and coating industry, most hydrocarbon emissions are trace-
able to the solvent in the coating material, all of which eventually evaporates. Our
purpose is to acquaint supervisory and management personnel in the industry with
methods of reducing the emission of organic solvents to the atmosphere and to help
them assess the costs. We will be as practical as possible and will present a number
of realistic options.
The logical sequence of steps toward achieving compliance suggests a division of
this publication into two parts.
Part A is concerned with reducing and controlling hydrocarbon emissions at their
in-plant sources. It includes background material on the nature of hydrocarbon emis-
sions and step-by-step information on plant surveys and emission control procedures.
Part B details the techniques available for end-of-line treatment of these emis-
sions. Because these techniques often involve the use of heat energy, methods for re-
covery of this heat will also be described.
This handbook is part of the U.S. Environmental Protection Agency Technology
Transfer Seminar Series for the Machinery and Mechanical Products Industry. A
companion publication discusses control of air pollution from metal cleaning processes,
and a third volume delineates water pollution control in the metal industry.
This publication, like the others in the series, provides practical, realistic op-
tions, based on the current literature and on the experience and know-how of people
throughout the industry.
111
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Part A
REDUCING HYDROCARBON EMISSIONS
THROUGH IN-PLANT PROCESS CHANGE
Part B
TREATMENT OF HYDROCARBON
EMISSIONS AND HEAT RECOVERY
May 1977
U.S. Environmental Protection Agency
Technology Transfer Office
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CONTENTS
Page
INTRODUCTION iii
PART A - REDUCING HYDROCARBON EMISSIONS THROUGH
IN-PLANT PROCESS CHANGE 1
Chapter I - PHYSIOCHEMICAL FACTORS AFFECTING HYDROCARBON
EMISSIONS IN COATING OPERATIONS 2
Definition of Organic Solvents 2
Evaporation Rates 3
Atmospheric Concentrations 3
TLV and LEL 5
Reactive Hydrocarbons 6
Calculations for Determining Reactivity of a Coating Formula 1°
Chapter II - PLANT OPERATING FACTORS AFFECTING
HYDROCARBON EMISSIONS 12
Non-processing Factors Affecting Emissions 12
Processing Factors Affecting Emissions 18
Chapter III - PLANT SURVEYS OF HYDROCARBON EMISSIONS 26
Obtaining the Regulatory Requirements 26
Determining Coating Operations To Be Regulated 26
Identifying Major Emitters 27
Estimating Amount and Type of Emissions 27
Measuring Emission Levels 27
Planning for Compliance 28
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Page
Chapter IV - EMISSION REDUCTION BY IN-PLANT PROCESS
CHANGE: OPPORTUNITIES AND PROBLEMS 29
Emission Control Through Formula Changes 29
Emission Control Through Process Changes 45
SUMMARY 48
PART B — TREATMENT OF HYDROCARBON EMISSIONS
AND HEAT RECOVERY 50
Chapter I — DISPOSAL OF SOLVENT VAPORS 51
Combustion 51
Vapor Adsorption 81
Chapter II - HEAT RECOVERY 82
Chapter III - COST OF COMBUSTION AND HEAT
RECOVERY SYSTEMS 92
SUMMARY 96
VI
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FIGURES
Figure
PART A
1 Typical Evaporation Rates 4
2 Comparison of TLV and LEL for Some Solvents 6
3 ARE Reactivity Classification of Organic Compounds 8
4 Solvent Emissions from Various Coating Formulas 13
5 Solvent and Resin Emissions from Typical Coating Formulas .... 14
6 Emissions from Coated Metal Parts or Assemblies 16
7 Potential Solvent Vapor Emissions from Coating Operations .... 17
8 Evaporation Rates of Various Formulas 21
9 Relative Costs of Coatings 30
10 Examples of Surface Coating and Added Thinner Formulas
on an As-Purchased Basis Having Conforming Solvent Systems ... 31
11 Energy Requirements for Comparable Operations 33
12 Solids vs. Viscosity for Caprolactone, Acrylic, and
Polyester Polyols 34
13 Examples of Modern Formulas for High-Solids Systems 35
14 Relative Emissions of a Hypothetical Waterborne System
Containing 20% Solvent and of a Conventional Solvent
Base System 36
15 Comparison of the Amount of Organic Volatile Material
Contained in High-Solids, Water-Soluble, and
Conventional Paints 37
16 Heat Requirements for the Baking of Equivalent Solvent-Borne
and Waterborne Coatings 39
17 Comparative Economics of High-Speed Curing Units 42
18 Comparative Costs of U.V. Curing and Infrared Curing 43
PART B
1 Coupled Effects of Temperature and Time on Rate of
Pollutant Oxidation 53
2 Typical Effect of Operating Temperature on Effectiveness of
Thermal Afterburner for Destruction of Hydrocarbons and CO ... 54
3 Available Heats for Some Typical Fuels 58
VI1
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Figure Page
4 Generalized Available Heat Chart for All Fuels at Various
Flue Gas Temperatures and
Various Excess Combustion Air ................. 59
5 Maxon Combustible Burner .................. 69
6 Hirt Multijet Gas Burner ................... 70
7 Afterburner Using a Discrete Burner .............. 71
8 Schematic of Catalytic Afterburner System ........... 72
9 Combustion Efficiency as a Function of Catalyst-Volume/
Flow Ratio .......................... 73
10 Typical Temperature-Performance Curves for Various Molecular
Species Being Oxidized Over Pt/Al2O3 Catalysts ......... 74
11 Effect of Solvent Concentration on Required
Preheat Temperature ..................... 75
12 Typical Shell and Tube Heat Exchangers ............. 85
13 Rotary Regenerative Heat Exchanger .............. 86
14 Heat Pipe .......................... 87
15 Process Heat Recoverable from Afterburner .......... 88
16 Capital Cost of Incineration .................. 93
17 Annual Variable Cost of Incineration .............. 94
TABLES
Table Page
I Combustion Constants ..................... 57
II Enthalpies of Gases Expressed in
Btu/scf of Gas ........................ 65
in Combustion Characteristics of Natural Gas ........... 66
IV Combustion Data Based on 1 Pound of Fuel Oil .......... 67
V Comparison of Heat Recovery Techniques ............ 83
Vlll
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Part A
REDUCING HYDROCARBON EMISSIONS
THROUGH IN-PLANT PROCESS CHANGE
-------
CHAPTER I
PHYSIOCHEMICAL FACTORS AFFECTING
HYDROCARBON EMISSIONS IN COATING OPERATIONS
The following topics are basic to the understanding and discussion of hydrocarbon
emissions and their control. They include:
• Definition of Organic Solvents
• Evaporation Rates
• Atmospheric Concentrations
• TLV and LEL
• Reactive Hydrocarbons
• Calculations for Determining Reactivity of a Coating Formula
DEFINITION OF ORGANIC SOLVENTS
Volatile organic substances have been defined in the Federal Register (CFR
52.1596, subsections (a) (i) and (k)) as follows:
"Organic materials mean chemical compounds of carbon excluding
carbon monoxide, carbon dioxide, carbonic acid, metallic carbides,
metallic carbonates and ammonium carbonates and having a vapor
pressure of 0.02 pounds per square inch absolute or greater at
standard conditions, including but not limited to petroleum fractions,
petrochemicals and solvents.
For the purposes of this section, organic solvents include diluents
and thinners which are liquids at standard conditions and which are
used as dissolvers, viscosity reducers, or cleaning agents."*
Although this definition was evolved for a specific region,** it is EPA's most re-
cent designation of the hydrocarbons to be controlled in maintaining acceptable atmo-
spheric burdens.
*'• Federal Register, Volume 38 - No. 218, November 13, 1973, pp. 31398-31399.
**For "New Jersey portions of the New Jersey, New York, Connecticut Interstate and Metropolitan Philadelphia
Interstate Air Quality Control Regions."
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EVAPORATION RATES
The vapor pressure of a chemical compound, central to its evaporation rate, re-
lates only to its temperature. It has been measured for most of the pure compounds;
tables of value are readily available for most normal plant circumstances.*
There are several empirical relationships between volatility (or relative evapora-
tion rate) and vapor pressure. According to Gaynes, "The simplest form for calcula-
ting relative evaporation rates is multiplying the molecular weights in question by the
vapor pressures. "**
For example, ethyl acetate has about the same vapor pressure as ethyl alcohol,
yet it evaporates twice as fast because its molecular weight is twice that of the alcohol.
This comparison is also true for butyl alcohol and butyl acetate or ethyl alcohol and
toluol.
Tysall and Wheeler state that "the rate at which a solvent evaporates from a film
is a technological measurement—combining the effect of a number of basic physical
properties such as vapor pressure, latent heat of evaporation and density of solvent
vapor, t
Doolittle presents typical evaporation rates for a series of solvent compounds by
comparing them to n-butyl acetate, which is arbitrarily given a rate value of l.tt
Figure 1 shows that the rates range from 1 to 3,000.
Finally, evaporation rate is influenced by air circulation over the surface of the
solvent coating; the higher the volume of air, the faster the evaporation.
ATMOSPHERIC CONCENTRATIONS
In discussing air pollution, we are naturally concerned with concentration levels
of pollutants. There are three main methods for calculating concentrations of solvent
vapors: partial pressure, volume, and milligrams per cubic meter.
PARTIAL PRESSURE
There is a general relationship between the amount of a solvent in the air and the
vapor pressure of this solvent. At any temperature a solvent will continue to evaporate
until the air becomes saturated, much as water evaporates to cause humidity. At a
normal atmospheric pressure of 760mm of mercury, the mixture of air and solvent
vapor will behave as if each exerted a pressure the total of which would be 760mm.
The solvent will exert its own vapor pressure, called partial pressure.
*For instance, see Chemical Engineers' Handbook. Perry et al., New York: McGraw-Hill.
**Gaynes, N. I., Metal Finishing, Volume 74, No. 1, Jan., 1976, p. 29.
t Tysall and Wheeler, The Science of Surface Coating. New York: Van Nostrand, 1962.
tfDoolittle, The Technology of Solvent and Plastici/ers, 1954.
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Methyl chloride
Isopropyl alcohol
n-Butyl acetate
Xylene
Dipentene
di-Ethylene glycol monethyl ether
Relative Evaporation Rate
with n-Butyl Acetate at 1 .0
27.5
3.0
1.0
0.63
0.08
0.01
Source: The Technology of Solvent and Plasticizers. Doolittle, 1954.
Figure 1. Typical Evaporation Rates
In a fixed volume of air, enough solvent will evaporate to create that pressure.
For instance, at temperature T, if a solvent has a vapor pressure of 200mm of mer-
cury there will be in that mixture of air and solvent vapor enough solvent to represent
rr- of the total volume. This also means that in a mole of the mixture there will be
— : mole of the solvent and •—— mole of air. If the solvent has a molecular weight of
760 7ou
92 and the air a molecular weight of 29, there will be 24.21bs. I — — x 92 of solvent
\760 /
and 21.41bs. 7- x 29 of air, for a total weight of 45.6 or a weight concentration of
53 percent solvent and 47 percent air. This is useful for calculating emissions meas-
ured in pounds of solvent per hour.
VOLUME
Another way of expressing the concentration of solvent vapor is by volume, stated
either in percent by volume or in parts per million (ppm) . In the case mentioned above ,
the concentration of the solvent vapors in percent by volume would be ( — — x 100
which is 26.3 percent. Expressed in ppm, the concentration would be 263,000 ppm.
MILLIGRAMS PER CUBIC METER
Finally, there is away, increasingly used, to express concentrations in milli-
grams per cubic meter (mg per m3). Using the above example, the concentration
would be 24.2gm (26.3% x 92) of solvent in 22.4 liters, which converts to 24,200mg
=1.08xl06mg/m3.
-------
The solvent used in all these calculations is highly volatile. In the case of a less
volatile solvent, xylene, the figures at 20°C would be approximately:
• Concentration by weight - 2.3%.
• Concentration by volume = 0.66% or 6,600 ppm.
• Concentration by mg/m3 = 693 x ^-^ - 31, 000 mg/m3.
These calculations assume conditions under which solvents can evaporate into fixed
volumes of static air and give the maximum concentrations under circumstances of
ideal equilibrium. In most industrial operations, air is moving so that equilibrium is
not achieved and actual concentrations are lower.
TLV AND LEL
There are two values of the air concentrations of a given solvent vapor that are of
considerable importance to industry: the threshold limit value (TLV) and lower explo-
sive limit (LEL).
• TLV relates to toxicity expressed in ppm and is an arbitrary value based on
physiological considerations. It represents the conditions under which it is be-
lieved that nearly all workers may be repeatedly exposed, day after day, with-
out adverse effects.*
• LEL, the lower explosive limit, represents a property of the vapor. It is the
lowest solvent concentration at which the mixture does not sustain combustion.
For insurance and for other obvious reasons, it is industry practice to provide
enough ventilation to maintain a solvent concentration well below this limit.
The usual value is set at 25 percent of the LEL. Explosive limits are usually
given in percent by volume; one percent is equal to 10,000ppm.
TLV, LEL, and 25-percent LEL values for some typical classes of solvents are
given in Figure 2. You will note that TLV's are much lower than even the 25-percent
LEL. The practical importance of this fact will be discussed later.
In the above discussion of concentrations, the equilibrium concentration of xylene
by volume was shown to be 6, 600ppm at 20°C. As shown in Figure 2, the TLV is
lOOppm and the LEL is 10,000ppm. This means that about 60 times more air than is
necessary for evaporation has to be supplied to comply with the TLV and about 2.5
times more to comply with the 25-percent LEL.
In general, TLV and LEL requirements demand much larger volumes of exhaust
air than are necessary from a strictly operational point of view.
*N. Irving Sax, Dangerous Properties of Industrial Materials. Fourth Ed., New York: Van Nostrand Remhold
Company, 1975.
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Toluene
Xylene
Isopropyl alcohol
Methyl-ethyl-ketone
n-Butyl acerafe
Merhylene chloride
TLV
200
100
400
200
150
500
LEL
13,000
10,000
25,000
18,100
17,000
None
25% LEL
3,300
2,500
6,300
4,500
4,300
None
Source: N. Irving Sax, Dangerous Properties of Industrial Materials. Fourth Ed., New York: Von Nostrand
Reinhold Company, 1975.
Figure 2. Comparison of TLV and LEL for Some Solvents (ppm/volume)
REACTIVE HYDROCARBONS
Because of the national scope of coating operations, it is expected that a federal
policy will be proposed through the Environmental Protection Agency. Major consum-
ers of hydrocarbon-based coatings, such as the automotive, coil, and can coating seg-
ments of the metal coating industry, will be a prime initial target for emission
guidelines. This could lead to guidelines for other high-volume repetitive coating op-
erations such as those for paper, textiles, wood, and adhesive laminations. To date,
there are no federal guidelines for hydrocarbon emissions from coating operations.
Meanwhile, many states and other political subdivisions have either proposed or
actually enacted legislation to limit atmospheric contamination by hydrocarbon emis-
sions. For instance, California, particularly the Los Angeles basin area, has pro-
mulgated Rules 66, 102, and 442, to be discussed shortly.
Industry personnel should become familiar with the terminology in existing regula-
tions so they can determine whether the solvents they use are likely to be affected by
later guidelines.
Solvents used in coating processes are classified according to their photochemical
reactivity. Briefly, photochemical reactivity, sometimes shortened to "reactivity,"
is "the tendency of an atmospheric system containing the organic compound in question
and nitrogen oxides to undergo, under the influence of ultraviolet radiation (sunlight)
and appropriate meteorological conditions, a series of chemical reactions that result
in the various manifestations associated with photochemical air pollution. These in-
clude eye irritation, vegetation damage and visibility reduction. "*
^Control Techniques for Hydrocarbon and Organic Solvent Emissions from Stationary Sources, AP.68, EPA.
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As a result of these definitions and because of severe local smog conditions, a
special regulation was issued to cover use in the Los Angeles area of materials con-
taining reactive hydrocarbons. This is "Rule 66," which has become a byword for
legislation on hydrocarbon emissions. In part, it reads:
"For the purposes of this rule, a photochemically reactive solvent is any solvent
with an aggregate of more than 20 percent of its total volume composed of the
chemical compounds classified below or which exceeds any of the following indi-
vidual percentage composition limitations, referred to the total volume of solvent:
(1) A combination of hydrocarbons, alcohols, aldehydes, esters, ethers, or
ketones having an olefinic or cyclo-olefinic type of unsaturation: 5 percent;
(2) A combination of aromatic compounds with eight or more carbon atoms to the
molecule except ethyl-benzene: 8 percent;
(3) A combination of ethyl-benzene, ketones having branched hydrocarbon struc-
tures, trichloroethylene, or toluene: 20 percent."
There has been considerable controversy, however, about the facts on which Rule
66 was based and especially about its applicability to areas other than the Los Angeles
basin.
The rule was eventually amended by two other rules of the Southern California Air
Pollution Control District:
1. Rule 102 changed the listing of solvents in Rule 66 by the following additions or
subtractions:
Type (1) Solvents — perchloroethylene is excluded.
Type (2) Solvents — methyl benzoate and phenyl acetate are excluded.
Type (3) Solvents — no change.
To aid industry in determining if specific coating formulas were in compliance, an
expanded tabulation was issued by the Air Resources Board (ARB) in Resolution 76-12
of February 19, 1976. It states:
"Now, Therefore, Be It Resolved, the Air Resources Board hereby adopts for
the purposes of inventory and planning, the classification of organic compounds
according to photochemical reactivity as set forth in Appendix V attached
hereto."
The Appendix V referred to in the resolution is presented here as Figure 3.
2. Rule 442, the second rule amending Rule 66, imposed specific limitations on
emissions. Note that these are not clear as to definition of a coating line or entire
coating plant, although subsection (b) uses the word "collectively. "
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Class I
(Low Reactivity)
Cj^-C Paraffins
Acetylene
Benzene
Benzaldehyde
Acetone
Methanol
Tert-alkyl alcohols
Phenyl acetate
Methyl benzoate
Ethyl Amines
Dimethyl formamide
Perhalogenated
Hydrocarbons
Partially halogenated
paraffins
Phthalic Anhydride
Phthalic Acids (2)
Acetonitrile^ '
Acetic Acid
Aromatic Amines
Hydroxyl Amines
Naphthalene (1^
Chlorobenzenes ^
Nitrobenzenes *• '
Class II
(Moderate Reactivity)
Mono-tert-alkyl-benzenes
Cyclic Ketones
Alkyl acetates
2-Nitropropane
C3+ Paraffins
Cycloparaffins
n-alkyl Ketones
N-methyl pyrrolidone
N,N-dimethyl acetamide
Alkyl Phenols (1)
Methyl phthalates (2)
Class III
(High Reactivity)
All other aromatic hydro-
carbons
(including partially halo-
genated)
Aliphatic aldehydes
Branched alkyl Ketones
Cellosolve acetate
Unsaturated Ketones
Primary 6 secondary C2+
alcohols
Diacetone alcohol
Ethers
Cellosolves
Glycolsdl
C2+ Alkyl phthalates^
Other Esters (?)
Alcohol Amines^)
03+ Organic acids + di acid (
03+ di acid anhydrides^2)
Formin ^ '
(Hexa methylene-tetramine)
Terpenic hydrocarbons
Olefin oxides ^
Phenol
(1)
(1) Reactivity data are either non-existent or inconclusive, but conclusive date from similar compounds are available;
therefore, rating is uncertain but reasonable.
(2) Reactivity data are uncertain.
Source: Communication from the State of California Air Resources Board, Appendix V, Resolution 76-12.
Figure 3. ARB Reactivity Classification of Organic Compounds
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"(a) A person shall not discharge organic materials into the atmosphere from
equipment in which organic solvents or materials containing organic solvents
are used, unless such emissions have bjen reduced by at least 85% or to the
following:
(1) Organic materials that come into contact with flame or are baked, heat
cured or heat polymerized, are limited to 1.4 kilograms (3.1 pounds)
per hour not to exceed 6.5 kilograms (14.3 pounds) per day.
(2) Organic materials emitted into the atmosphere from the use of photo-
chemically reactive solvents are limited to 3.6 kilograms (7.9 pounds)
per hour, not to exceed 18 kilograms (39.6 pounds) per day, except as
provided in subsection (a) (1). All organic materials emitted for a dry-
ing period of 12 hours following their application shall be included in this
limit.
(3) Organic materials emitted into the atmosphere from the use of non-
photochemically reactive solvents are limited to 180 kilograms (396
pounds) per hour not to exceed 1350 kilograms (2970 pounds) per day,
except as provided in subsection (a) (1). All organic materials emitted
for a drying period of 12 hours following their application shall be in-
cluded in this limit.
(b) Equipment designed for processing a continuous web, strip or wire which
emit organic materials shall be collectively subject to the limitations stated
in subsection (a).
(c) Emissions of organic materials into the atmosphere required to be controlled
by subsection (a) shall be reduced by:
(1) Incineration, provided that 90 percent or more of the carbon in the or-
ganic material being incinerated is oxidized to non-organic materials,
or
(2) Incineration, provided that the concentration of organic material follow-
ing incineration is less than 50ppm, calculated as carbon and with no
dilution, or
(3) Adsorption, or
(4) Processing in a manner determined by the Air Pollution Control Officer
to be not less effective than (1) or (3) above. "*
Communications from State of California Air Resources Board, July 26, 1976.
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CALCULATIONS FOR DETERMINING REACTIVITY OF A COATING FORMULA
Each coating formula containing solvents may have to be revised according to Rule
66 or Rule 102. For determining if revision is necessary to achieve conformity, use
the calculations that follow.
For evaluating solvents in connection with Rule 66:
Given: A coating solvent with the following composition:
Toluene
Xylene
Methyl isobutyl ketone
Isophorone
Saturated aliphatic solvents
Total
15.0%
2.0%
7.0%
10.0%
66.0%
100.0% by volume
Problem: To determine if this solvent system is photochemically reactive as
defined by Rule 66.
Solution: Tabulate the materials in the solvent that may be photochemically
reactive. Columns (1), (2), and (3) refer to the photochemically reactive
groupings listed on page 8.*
Chemical Name
Toluene
Xylene
Methyl isobutyl
ketone
Isophorone
Aliphatic
solvents
Total
Limit
Classification
Name
Aromatic
hydrocarbon
Aromatic
hydrocarbon
Branched alkyl
ketone
Cyclic ketone
C, + paraffins
(1)
0 %
0
0
10.0
0
10.0%
5.0%
(2)
0 %
2.0
0
0
0
2.0%
8.0%
(3)
15.0%
0
7.0
0
0
22.0%
20.0%
*Readers may need a chemical handbook to relate these compounds-and those used in their plants-to the
classifications in Columns (1), (2), and (3) of Rules 66 and 102.
10
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This system is photochemically reactive on three counts:
• The group (1) total exceeds the 5 percent allowance.
• The group (3) total exceeds the 20 percent allowance.
• The total of all groups (34 percent) exceeds the 20 percent total allowance.
Utilizing the expanded ARE tables and definition of Rule 102, the positioning of
various solvents changes as follows:
' Chemical Name
Toluene
Xylene
Methyl isobutyl ketone
Isophorone
Aliphatic solvents
Total
(1)
0.0%
(2)
10.0%
66 . 0%
76.0%
(3)
15.0%
2.0%
7.0%
24.0%
The significant differences between Rule 66 and Rule 102 (plus the ARE tables) are
the movement of some solvents into higher reactivity categories and the inclusion of
aliphatic solvents as part of the reactivity calculation.
11
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CHAPTER II
PLANT OPERATING FACTORS AFFECTING
HYDROCARBON EMISSIONS
The first part of this chapter will be a discussion of emissions from various
coating formulas and thicknesses.
In the second part, we will examine the steps of the coating process to see what
each contributes to the total emission picture.
NON-PROCESSING FACTORS AFFECTING EMISSIONS
The amount of organic emission is related to:
• Composition of the coating;
• Amount of coating applied;
• Post-application chemical changes; and
• Non-solvent contaminants.
COMPOSITION OF THE COATING
As we have seen, the amount of solvent emitted depends on the composition of the
coating material.
Figure 4 shows in very general terms the amounts of solvent emitted under the
same conditions from various coating compounds. In general, low-solids lacquers
will produce more emissions than high-solids urethanes, and significantly more than
waterborne systems.
Some typical values of solvent emissions in grams per square meter for different
coating systems are given in Figure 5. The five enamels shown in the figure, which
contain from 29 to 57 percent solvent, will emit 52 to 79 grams per square meter.
Note that there will also be some emissions from unreacted resin components and de-
composition products that volatilize during baking.*
*Resiri emissions generally come from thermosetting coatings that require polymerization or crosslinking of low
molecular weight fractions. These components gradually build in molecular weight (with decreased volatility) as
the exposure time and temperature increase. Hence, the emissions contain both solvent and polymer fractions.
12
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I
CD
5 50 —
CJ
X
>-
CO
o
CO
0
UJ
Q.
DISPERSION
LACQUER
NONAQUEOUS
DISPERSION LACQUER
NONAQUEOUS
DISPERSION ENAMEL
HIGH SOLIDS URETHANE
32 percent SOLIDS, WATER-BORNE SPRAY
40 percent SOLIDS, WATER-BORNE SPRAY
HIGH SOLIDS (80 percent SOLIDS)
50 percent SOLIDS, WATER-BORNE SPRAY (80% water/20% solvent)
ELECTRODEPOSITION (PRIMING ONLY)
POWDER COATING
5 10 15 20 25 30 35 40 45 50 55
POUNDS OF ORGANIC SOLVENT EMITTED PER GALLON OF SOLIDS APPLIED
60
Source: Private Communications.
Figure 4. Solvent Emissions from Various Coating Formulas
13
-------
Primer Primer
Vinyl Primer Zinc
Enamel, Enamel, Enamel, Acrylic Alkyd Zinc Primer Zinc Vinyl Epoxy Vinyl
Air Dry Baking Dipping Enamel Enamel Chromate Surfacer Chromate Chromate Polymide Vinyl Acrylic Polyurethane
Solvent
5703
4323
2912
.5242
.4683
.3872
2150
.4343
5473
.4029
.5811
7588
.5071
Aliphatic
Aromatic
Saturated
Ketones
Saturated
Saturated
Alkyd
Vinyl
Acrylic
Epoxy
Urethane
Cellulosic
Ammo
Solvent Emissions Emission Factor g/m2*
hydrocarbons Mineral spirits 66 2 56 0 28.4 56.6 11.0 16 7 59.3
hydrocarbons Toluene 4.4 75 34 5.5 2,1 5.3 56 5.0
alcohols n-Propyl alcohol .4 13.4 11 8 10 8.4
Methyl ethyl 60.2 9.0
ketone
esters n-Butyl acetate 1 7 10.2 9 9 10.3
esters Ethylene glycol 12.4 3.1
mono methyl
ether
Resin Emissions
Phthahc anhydride 38 46 50 40 1.6
Vinyl chloride 1.5 18
Methyl methacrylate 1.4
Epichlorohydrin 15 5
Toluene diisocyanate
Methyl ethyl ketone
Ethanolamme
13 8
6.0 14.4 16 7 14.8
17.5 77.9
42.2 22 9 66.7 13.9 9.7
14 0 50.8
12.9
1.4 1.5
1.2
4.6
5.0
Resin ester Maleic anhydride
Styrene
Phenolic
Styrene
Phenol
Hydrocarbon Turpentine
Total 74 4 68 5 51.9 78.8 62.7 61.0 52 2 74.5
76.3 73.8 84.9 93 0 79.9
*At typical coating thicknesses
Source Private Communications
Figure 5. Solvent and Resin Emissions from Typical Coating Formulas
-------
Emissions from thermoplastic coatings are almost totally solvent. These poly-
mers are applied at high molecular weight, so that little change occurs between appli-
cation and baking.
Emissions can also be linked to the nature of the part to be finished. Expected
levels of emissions per-unit-produced are given in Figure 6 for some segments of the
metal coating industry. Although total emissions vary significantly from a beverage
can to a washer, due to the area covered, the net emissions per square meter are
about the same.
Obviously, two factors must be considered in selecting coating formulas: the type
of material to be coated and the characteristics of the desired finish. In a later sec-
tion, we will discuss how recent developments in formulation have increased the choice
of formulas, with particular significance for overall emission reduction.
In addition, an economic choice sometimes has to be made in meeting emission
standards: whether to invest in emission control equipment or switch to a more expen-
sive coating. This will be discussed later.
AMOUNT OF COATING APPLIED
The total emissions during a coating process are affected by:
• The area to be coated;
• The thickness.of the coat;
• The efficiency with which it is applied; and
• The percent of solvent in the coating.
Area and thickness can be controlled to some extent by design of the part and the
application technique.
Application efficiency—for instance, avoiding overspray, overcoating, and excess
widths and using a minimum of passes to achieve the thickness desired—can profoundly
influence total emissions and is directly controllable by operating personnel.
Figure 7 gives a simple equation for predicting the total amount of solvent emis-
sion from any operation involving non-waterborne coating.
POST-APPLICATION CHEMICAL CHANGES
Evaporation rates of individual solvents in the coating can vary to such an extent
that if significant air drying occurs before baking—generally the case—the solvent mix-
ture remaining in the coating at the beginning of the baking operation is much richer in
the high-boiling solvents. For example., a solvent mixture initially consisting of equal
parts of isopropyl alcohol and xylene will tend to lose isopropyl alcohol faster than
xylene. Thus, after air drying, which would remove most of the isopropyl alcohol, the
solvent to be removed in baking could be mostly, if not entirely, xylene.
15
-------
'Type of
..Hydrocarbon
Proia,t
Metal '"!in^ , Fxt i Beverage
Beverage Cans
Ductwork
Canopies and Awnings
Refrigerators
Screening
Fencing
Enamelrd Plumbing Fixture^
Diyers
Washers
Metal Doors, Excl Garage
Gutters
AhphVK, Aromatic Saturated
vdrocarbon:- Hydrocarbons Alcohols Retimes
J112
1556
529
1
1482
127
336
250
63
3112
95h 785 166 2 65"
956 785 166 2 657
n 2556 0 53>l 7 8650 0
0 1278 n 26M 8 4325 11
2 111 2 79 7 12H i)
38 1 13 24 3 83
0 1217 0 257 0 Jill 0
7 2(1 7 158
0 107 8 52 2 45 2
8 30 5 38 9 33 7
0 51 7 109 1/51
[1 255fc 0 539 o 8H50 0
S.Uur itecl Saturated
Total
Grams
':r*thane
.00?
fin 5
IP fin
* 4
007
8 0
Fpoxy
13 6
Epoxy
10 2
3
16 8
Ammo Per I'n
012 5 ,
042 5
13(. all 16334
68 2 8166
1041
6 7
«5 0 7778
200
687
513
2 8 330
!36 5 16334
it
017
017
0
8
3
860
n
6
7
3
G
0
Size
Gram
Unir Emissions
M2
OK45/can
OP45/can
231/tonne
105/ton
9 91/unlt
1/m2
110/tonne
1 97,/unit
6 38/umt
4 76/unit
4 32/unit
231/tonne
/M2
77
77
70
77
105
7
70
101
107
86
76
70.
8
8
7
8
25
9
7
8
8
8
5
7
NOTE In Metric Tons
Source1 1-nvironmental Protection Agency, Source Assessment Pnorinzalion of Air Pollution from Industrial Surface Coating Operations
LPA-650/2-75-01 9-9, February 1975.
Figure 6. Emissions from Coated Metal Parts or Assemblies
-------
w
_ 0.0623 A n (1-0.0.IP)
P f
where:
W = weighf of solvent vapors in Ib.
A = area coated (sq . ft.)
n = dry mils.
P = percent solids by volume
f = efficiency factor (dimensionless)
empirically determined (f < 1)
P - solvent density (Ib ./gal.)
Source: Foster D. Snell, Inc.
Figure 7. Potential Solvent Vapor Emissions from Coating Operations
Normally, however, the temperatures used in baking ovens are not high enough to
cause chemical changes in the remaining volatile solvents, except for the polymeriza-
tion of certain liquid monomers or oligomers (styrene, for instance).
In direct-heated, gas-fired recirculating ovens, widely used in the industry, cer-
tain changes may occur in the solvent through contact of the vapor-laden air with the
heating flame. Some of the solvent may be directly burned to carbon dioxide and water;
some may remain in its original chemical state; and some may be modified or chemi-
cally changed.
These modified chemical structures will result in emissions drastically different
from those expected from the initial composition of the solvent mixture; however, they
are still considered hydrocarbons, subject to emission controls and guidelines. They
can also contribute to the formation of tarry aerosols or condensables that constitute a
new component of the exhaust. Although these latter compounds are potential pollutants,
they have not been covered in any hydrocarbon emission guidelines published so far.
They also present potential surface-fouling problems for incinerators, adsorbers, or
heat-exchange equipment.
On the other hand, since a good proportion of the solvent vapors are probably
burned to CO2 and water in the combustion area, actual amounts of organic materials
emitted to the atmosphere may be less than that calculated by the standard mass bal-
ance methods.
Because of the changes that can occur during air drying and baking, stack sampling
must be done to obtain the actual emissions for design and compliance purposes.
17
-------
OTHER CONTAMINANTS
Coating operations produce other contaminants besides the solvent vapors. A ma-
jor source of these is the spraying operation. We will discuss over spray and ways of
reducing it later. For now, the relevant point is that part of the aerosol from the
spray gun may dry before it reaches either the target article and/or whatever device
(baffle or water curtain) has been set up to catch the wet overspray.
This dry material, in the form of particulates, will be part of the vapor exhaust.
In general, the amounts thus generated are not significant. However, if adsorption
devices are used for controlling the vapor emissions, the particulates will have to be
filtered out because they tend to foul the activated carbon beds.
PROCESSING FACTORS AFFECTING EMISSIONS
There are a number of commonly used coating processes. A brief survey of these
will be followed by discussion of emissions associated with the various processing steps.
COMMONLY USED COATING PROCESSES
The basic processes used for coating include spraying, dip coating, flow coating,
coil coating, and masking.
Spraying
Typical spraying operations are performed in a booth, with a draft fan to prevent
explosive or toxic concentrations of solvent vapors. Essentially, there are three
spraying techniques: air atomization, airless atomization, and electrostatic.
Air atomization uses its own air source, which may be heated, filtered and/or
humidified, or treated in some other fashion. Airless spraying, on the other hand,
atomizes without air by forcing the liquid material through specially designed nozzles
under a pressure of 1,000-2, 000psi. On release to atmospheric pressure, some
of the solvents in the surface coating vaporize and join with the straight hydraulic
forces at the nozzle as atomizing agents. In general, with airless spraying less sol-
vent will be volatilized in the spray booth than with air spraying, meaning that more
solids may have to be removed later during air drying or baking.
During airless atomization, total volatilization of a portion of the solvent will
probably occur, and emissions from this type of booth will be similar to the solvent
formula. Air atomization, which is based on partial volatilization of the solvent blend,
is likely to produce emissions high in low-boilers.
Electrostatic spraying projects charged coating particles into an electrostatic field
created by a potential difference of about 100,000 volts between the articles sprayed
and spray grids 12 inches away. The particles of wet paint from the spray gun enter
this field with the same potential as the grids and are thus repelled by them and at-
tracted to the article being sprayed.
18
-------
Dip Coating
In dip coating operations, the object is immersed for the required time in a tank
of the coating. When the object is removed, excess coating drains back into the tank,
either directly or via a drain ramp.
Flow Coating
Articles that cannot be dipped due to their buoyancy, such as pressure bottles, are
subjected to flow coating. Material is fed through overhead nozzles in a steady stream
over the article. Excess coating drains by gravity from the coated object and is re-
circulated. Removal of excess coating material and solvents is aided by jets of heated
air.
Coil Coating
Coil coating is a technique for applying finishes to long flat strips or coils of metal,
on one side or both, by means of rollers similar to those in a printing press. Three
power-driven rollers are normally used. One of the rollers is partially immersed in
the coating material. The coating is then transferred by direct contact to a second
parallel roller. The object to be coated is run between the second and third rollers
and is coated by the second roller.
Masking
Masking is a technique for applying coatings where sharp, clean edges are needed;
for instance, for lettering, stripping, and two-color finishes. The areas to be left un-
coated are masked with cloth, plastic sheeting, tape, or a special mask derived by
photography from an artwork pattern (silk screening).
The coating may then be applied by stencil or rubber squeegee. Masking is usually
removed while the coating is still wet to prevent frayed edges and to ensure sharpness.
EMISSIONS FROM THE VARIOUS PROCESSING STEPS
Emissions of solvent vapor vary not only with the coating formula but also with
each individual processing step.
Spraying
Paint-spray booths generally have one side open to the rest of the plant; ventilation
of the booth is necessary to ensure both operator and plant safety. Normally, spray-
booth ventilation velocities of from 100 to 150 feet per minute per square foot of booth
opening are adequate for manual operations.* Insurance standards require that the
*:AirPollution Engineering Manual, AP-40, 2nd Ed., U.S. EPA, May 1973.
19
-------
average velocity over the open face of the booth be not less than about 1.5 feet per sec-
ond. All fumes should be vented through a fume hood instead of into the plant.
Discharge from a paint-spray booth consists of particulate matter and organic sol-
vent vapors. The particulate matter consists of entrained coating material that did not
adhere to the object being painted or to the inside surfaces of the booth. The organic
vapors are generated from the evaporation of solvents, resins, diluent, or thinner.
Generally, emission levels are increased by overspray; that is, material that
misses the surface to be coated. The table below gives overspray percentages for the
various spray techniques.
Overspray Percentages as a Function of
Spraying Methods and Surfaces Sprayed*
Method of Spraying
Air atomization
Airless
Electrostatic
Disc
Airless
Air atomized
Flat
Surfaces
(%)
50
20 to 25
5
20
25
Table Leg
Surface
(%)
85
90
5 to 10
30
35
Bird Cage
Surface
(%)
90
90
5 to 10
30
35
* Air Pollution Engineering Manual, AP-40, 2nd Hd., EPA, May 1973.
Solvent concentrations in spray booth effluents vary from 100 to 200ppm for man-
ual operations. Solvent emissions from spray booth stacks vary with the extent of the
operation, from less than 1 pound to more than 3,000 pounds per day. No definitive
data is available for automatic spray booths.
Virtually all solvents evaporate in the course of the coating sequence, each at its
own rate. For measuring purposes, this evaporation is viewed in terms of "flash-off."
defined as the quantity of solvent evaporated under either ambient or forced conditions
from the surface of a coated object during a specific time. The graph in Figure 8
shows flash-off times for various coating types applied by spraying and is useful for
determining potential emissions from different coating systems. The total emission
load, however, is significantly affected by the size, shape, and number of pieces being
coated and other factors .
-------
100
90
80
- 70
c
s
! 60
CO
8 50
t—
| 40
o
CO
30
20
10
1. LACQUER: CLEAR, SEMI-GLOSS, FLAT, PIGMENTED, PRIMERS, PUTTIES, SEALERS
VINYL ORGANISOLS, STRIPPABLES, SOLVATED POLYESTERS
SOLVA1 ED VINYL PLASTISOLS
STAINS: SPIRIT, OIL
VARNISH: CLEAR AND PIGMENTED
ALKYDS. ACRYLICS, POLYURETHANES
EPOXIES
I
I i I
I
I
I
I
I
I
3 4 5 6 7 8 910
minutes
20
40
60
1 hr
2 hr 3 hr 4 hr 6 hr 8 hr 12 hr16 hr
TIME
Source: Air Pollution Engineering Manual. AP40, 2nd Ed., EPA, 1973.
Figure 8. Evaporation Rates of Various Formulas
Solvent emissions, then, vary with types of spray operations. However, particu-
late matter, the other type of emission, can be effectively removed (50 - 98 percent)
by techniques to control the particulate emissions. These include:
• Dry Baffle. In this method, the wet overspray collects on large panels called
baffle plates, which catch 50 - 90 percent of the particulates produced by spray-
ing a high-solids enamel. With low-solids lacquers containing highly volatile
solvents, efficiencies may be much lower due to the rapid drying of the lacquer
and poor adhesion of dry particles to the baffle.
• Paint Arrester. Filter pads used in this method can remove up to 98 percent
of paint particulates. Filtering velocities should be less than 1.3 m/sec.
• Water Wash. Water curtains and sprays are 95 percent effective in removing
paint particulates. A water circulation rate of 1 - 5 liters per cubic meter of
exhaust air is usually recommended. Surfactants may be added to the water to
aid in removing paint from the circulating tank.
In order of effectiveness, the paint arrestor would be considered the best technique
for removing particulates when downstream solvent vapor processes such as catalytic
or other afterburners, heat exchangers, or carbon absorption beds are used. Water
washing to remove particulates would be a second choice, assuming that the solvent
vapor processes can tolerate some water in the vapor stream. Baffle plates would be
considered the third and least effective method, although by far the cheapest.
21
-------
Concentrations of water-soluble solvent vapors are sometimes reduced, particu-
larly in non-recirculating sprays, however, this creates a water contamination prob-
lem necessitating treatment. Solvents tend to increase the BOD (biochemical oxygen
demand) level of wastewater to a considerable degree (several hundred ppm).
The following table shows the effectiveness of a water curtain in reducing solvent
vapors in a sample spraying operation:
Emissions from Automatic Airless Spraying Operations
(Alkyd Coating with Xylol Solvent)
Operation
Spray (no water curtain)
Spray (water curtain on)
Emissions in lb./hr.*
Particulate
0.5
0
Organic
4.0
3.5
These emissions total about 60 percent of the organic emissions from this particular operation. Typically, spraying
accounts for 40 - 60 percent of the total emissions from a coating operation.
Source: Foster D. Snell, Inc.
Note that in the case above the water spray reduced organic vapors discharged to
the atmosphere by about 10 percent. The contaminated water was collected and the
xylene recovered by separation, with the balance discarded. With highly soluble sol-
vents, for instance methyl-ethyl-ketone, distillation may be necessary to recover the
solvent and minimize sewer disposal.
Although air pollution was significantly reduced in this case, the disposal of sol-
vent or particulate-laden water to the sewer had to be carefully monitored to keep it
within water pollution guidelines. It is important, of course, to avoid substituting one
set of pollution problems for another.
Flash-off occurring after the spray operation but before baking is treated later in
this book as a separate category of emissions; Rule 66, however, includes pre-baking
emissions as part of spraying.
Other Application Techniques
Emissions from other application techniques such as flow coating, dip coating, or
coil coating differ from spray coating emissions to the extent that these methods re-
quire less coating material. However, the expected solvent emission load from these
techniques can vary widely.
In fact, flow coating may not be much better from an emission standpoint than
spray coating. For flow coating, the proper percentage of solids and correct viscosity
must be maintained. Further, so much solvent is lost during recirculation and air
22
-------
blow-off of excess coating that flow coating is often done in a "tunnel" to keep the
solvent-laden air in a fixed area. The result is that a well-run flow coating operation
using 60,000 gallons of coating per year may use as much as 54,000 gallons of makeup
solvent to compensate for "tunnel solvent" losses. This is much more wasteful than
an air-atomized spray operation with 50 percent over spray.
Dip coating solvent losses are generally under 10 percent, depending on time of
year and temperature in the plant. This usually represents much less solvent loss
than that occurring with spraying or flow coating and does not normally require much
makeup solvent.
From the standpoint of overall emissions, the single most efficient coating method
is roller or coil coating, a process in which extraneous evaporation is practically neg-
ligible, since all coating supplied to the coating head is placed onto the web to be coated.
Pre-Drying Processes
Enough solvent must evaporate before the coated part enters the finishing or curing
oven to avoid bubbling, uneven coating thickness, and other adverse effects. Along
with solvent evaporation, the pre-drying process allows time for the coating to level
itself if it has been unevenly applied. The skilled coating formulator can often vary
solvent balances to minimize these problems, as well as to reduce emissions from the
pre-drying operation.
Pre-drying is usually carried out on conveyors, which are often open to the atmo-
sphere. As will be discussed later, it may be advantageous to enclose these conveyors
to maintain the highest permissible vapor concentration in the air surrounding the dry-
ing parts. This allows a gentler drying of the coating to help prevent blisters or bub-
bles in the curing oven. Care must be taken, however, to ensure that the atmosphere
in the oven is in keeping with LEL determinations.
Emissions from pre-dryers will, in general, contain higher concentrations of the
low-boiling components of the solvent blend.
Ovens
The last step in coating operations is the final conditioning of the coating. While
certain coatings can be totally air dried, this is usually too slow for industrial proc-
esses. In general, heat must be applied to speed the curing rate.
A distinction can be made between drying and baking. Drying generally refers to
removal of volatiles such as solvents. Baking is the process by which a coating cures
or otherwise changes to develop its film integrity. However, this distinction has less
effect on emissions than the methods used and the type of oven.
There are two basic types of ovens: continuous and batch.
From an emission standpoint, the difference is important only insofar as the at-
mosphere of a batch oven is easier to control than that of a continuous oven. However,
23
-------
solvent evolution in a batch oven is a function of time and temperature, meaning the
coated part generally reaches the temperature required for baking the finish, making
subsequent handling difficult.
In a continuous oven, the evolution of solvent vapors varies in different zones of
the oven. This may enable more control, depending on the configuration of the oven's
exhaust system. In general, emissions in continuous ovens are more diluted than
those inibatch ovens, reducing problems with LELs. However, this dilution can make
emission control in exhaust gases from continuous ovens more difficult and expensive.
Ovens also differ in the way they provide heat. Oven design should allow for:
• Sufficient time before contact with heat for the coated surface to level and for
highly volatile solvents to evaporate slowly, inhibiting bubble formation;
• An initial low-temperature zone for continued slow evaporation of solvents, to
further inhibit bubbles;
• Sufficient time and temperature for a full cure of the coating;
• Termination of the heating process before the coating is damaged;
• A cool-down zone to set the coating and enable handling;
• Removal of emissions to prevent their interference with the curing process;
and
• Enough air flow to keep the atmosphere at approximately 25 percent of LEL,
well below the explosive limit, to be maintained by control of coating formula-
tion, air flow rate, and other variables.
Along with the basic design of a curing oven, a choice of heat source must be
made. This may be dictated by both the fuel or energy available and the emissions
expected. Types of oven heating include:
• Direct-fired gas heat, in which the products of combustion combine directly
with the process air. Oven burners may use either fresh makeup air or re-
circulated oven gases containing evaporated solvents and other volatiles.
Flame contact with recirculated gases may cause molecular cracking or con-
version, which may render the effluent gases photochemically active.
• Indirect-fired gas heat, in which combustion products pass through one side of
a heat exchanger and discharge directly into the atmosphere. Process air,
heated before being circulated to the oven, passes through the other side.
• Electrical heat, in which fresh makeup air or oven gases are passed over elec-
trical resistance or infrared heaters. This is similar to direct gas-fired heat,
but it eliminates combustion products. However, some solvent modification
can result from contact with the heating elements when resistance heaters are
used.
24
-------
In general, electrical heating costs more than direct-fired gas, but total emission
loads are reduced.
Baking or curing ovens can produce a variety of pollutants in addition to pure
"emissions" from the coating including (a) smoke and other products of incomplete
combustion resulting from improper operation of a gas- or oil-fired combustion
heating system, which can interfere with stack sampling procedures by fouling test
elements; and (b) aerosols arising from the partial oxidation of organic solvents ex-
posed to flame and/or high temperatures and from chemical reactions that occur in the
resins (these can be deposited on heat exchangers, adsorption beds, and related hard-
ware, reducing their effectiveness).
Emissions from ovens, therefore, vary significantly with the oven type (batch or
continuous), method of heating, condition of the part before it enters the oven (pre-
dried), and oven-operating parameters such as the allowable LEL.
Emissions from the Overall Coating Process
In most coating operations, 40 - 80 percent of the solvent evaporates at the time
of application and/or during subsequent air drying. The remaining 20 - 60 percent
evaporates in the oven.
The table below provides an overview of this chapter and gives general emission
ranges as a percentage of the total emission load from typical coating operations.
Percent of Total Emissions from Various Coating Steps
Coating Method
Spray Coat
Flow Coat
Dip Coat
Roller Coat
Coating Step
Application
30-50
30-50
5-10
0-5
P re /Air Dry
10-30
20-40
10-30
10-20
Bake
20-40
10-30
50-70
60-80
Source: Foster D. Snell, Inc.
In a specific example, 30 percent of the emissions occurred during the spray
process itself and another 8 percent occurred in the conveyor between the spray booth
and the continuous curing oven.
25
-------
CHAPTER III
PLANT SURVEYS OF HYDROCARBON EMISSIONS
An important early step in controlling emissions is to determine the volumes gen-
erated and their sources. Plant operators need to know which operations are most
responsible for solvent pollution. Identifying emissions from each process is essential
to developing a central plan for complying in a cost-effective and practical manner.
The plant survey gives this information and provides a basis for determining if the
plant is in violation of regulations and to what degree. To carry out an effective plant
survey the following steps are necessary:
• Obtain the latest regulatory requirements;
• Determine which coating operations are affected by the regulations;
• Determine which coating operations are major emitters;
• Estimate the emissions from the sources identified;
• Measure the level of emissions; and
• Develop a plan to minimize emissions and improve the plant's compliance
position.
OBTAINING THE REGULATORY REQUIREMENTS
Most cities and states have Air Pollution or Air Control Offices. Contact the one
in your area for the latest regulations that may affect the operation of your plant. Dis-
trict offices of the U.S. Environmental Protection Agency should also be asked for any
relevant information.
There is always a tendency to let sleeping dogs lie and avoid involving local regu-
latory agencies if at all possible. It is better, however, to be aware of existing and
potential regulations and guidelines as they are promulgated. One reason is that con-
struction and operating permits are required for any equipment causing emissions, and
states keep records of these in order to later implement air pollution control plans.
DETERMINING COATING OPERATIONS TO BE REGULATED
Since emission standards vary from one area to another, a coating line may be in
compliance in one state and in violation in another. Opportunity may thus exist, over
the short term, for a company to increase production in a plant bound by less stringent
26
-------
emission standards, allowing time for bringing all coating lines into compliance
without a loss in production.
IDENTIFYING MAJOR EMITTERS
Coating operations should be assessed in terms of their overall contribution to total
plant emissions. Large volumes of solvents used to clean applicator rolls contribute to
solvent emissions, yet are not generally included in coating-line solvent calculations.
Spray booths used intensively but for short periods of time must be considered. "Tun-
nel losses" from flow coating lines contribute significantly to emissions. A noncoating
operation, such as panel degreasing, may be an important emitter. Finally, if a plant
makes its own coating, this operation can also be a major emitter.
ESTIMATING AMOUNT AND TYPE OF EMISSIONS
Once the plant has determined the major sources of emissions, an overall tabula-
tion should be made of the amounts and types. This tabulation must include solvents
used for makeup, dilution, and cleaning.
Coating suppliers should be contacted to find out the percentage of solids and types
of solvents in their products. This also serves notice to the supplier that the plant is
interested in compliance-type solvents.
Ideally, each article would be coated and then weighed immediately after both air
drying and baking to determine how much weight loss (emission) takes place at each step.
Based on these weights, and on the temperatures of drying and baking and the formula-
tion supplied by the coating manufacturer, an estimate could be made of the type of
emissions from each stage of the process. Obviously, this would not be practical for
auto bodies, refrigerator paneling, and other large items. Sample coupons or small
panels might be interjected into the coating line to obtain the information for large
pieces, however.
As a first approximation, the daily consumption of coating multiplied by the per-
cent of solvent would produce a total solvent emission load. This total load would then
be factored according to the breakdown in the table on page 26, presented to show per-
centage of emissions from individual process steps. As stack testing is expensive,
some states accept the results of such material-balance calculations.
MEASURING EMISSION LEVELS
The only reliable method for determining actual emissions is to measure them in
the effluent streams. The major effluent stream for gaseous emissions is the stack,
which transports the final emissions after the stream has passed through paint arres-
tors, water wash towers, adsorption devices, catalytic afterburners, etc.
27
-------
However, the validity of this method, relied on more and more by regulatory
agencies, is impaired because of the following factors:
• Measurements are based on volumetric quantities, which are significantly af-
fected by temperature;
• Many of the analytical techniques commonly used do not give a real value for
the amount of material in a given volume of gas, and empirical factors have to
be applied;
• Variations in air flow and/or concentrations are difficult to compensate for
with current equipment; and
• Some of the emissions may be compounds for which no standard analyses are
available.
Stack sampling results are also affected by the point at which the sample is taken.
Although continuous operations would tend to produce a uniform level of emissions,
batch operations can produce constantly changing emission loads. This means that for
a total picture of a given plant's operation, continuous monitoring is probably required.
A further problem with stack sampling is that, in general, emissions from a plant
or coating line are discharged through more than one stack. Therefore, each has to
be monitored, unless the exhaust can be combined before sampling.
PLANNING FOR COMPLIANCE
Once it has been determined that a certain coating operation is the major emitter,
steps should be taken to reduce its emission load by formula changes, process modifi-
cations, or other means.
This should be followed by effective policing to ensure that the changes are, in
fact, producing the desired emission reduction. The second major emitter should then
be approached in the same manner.
In any comprehensive survey and action program, the services of outside experts
may be worthwhile. Experienced consultants have an up-to-date awareness of current
regulatory thinking, without preconceived biases as to how the regulations should be
approached or applied. They have access to the latest technology in stack sampling
procedures, which can shorten the training period for plant personnel. Finally, con-
sultants, using the plant's stack analyses and their familiarity with the regulations,
can advise plant managers how compliance may best be achieved.
28
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CHAPTER IV
EMISSION REDUCTION BY IN-PLANT PROCESS
CHANGE: OPPORTUNITIES AND PROBLEMS
Earlier in this publication, the various approaches to emission reduction were
broadly presented. In this chapter, we will discuss technical and economic aspects of
formula changes and the potential impact of process changes.
EMISSION CONTROL THROUGH FORMULA CHANGES
The problem associated with formula changes can be both technical and economic.
Before discussing these problems, we assume that experimentation has been or will be
performed to ensure that the new coating meets predetermined specifications, that ad-
equate supplies of the coating are available, and that plant personnel are fully trained
in its application. Finally, the revised coating must be checked at the outset against
internal cost standards, a point illustrated by the sharply varying costs of the polymer
systems in Figure 9. The data, although 8 years old, also illustrates the wide variety
of coating systems available. Note that silicone and fluorochemical polymers are still
the most expensive.
The main ways of varying formulas are discussed below, in terms of both advan-
tages and problems.
SOLVENT CHANGES
As a result of regulations affecting the use of photochemically reactive solvents,
practically all the conventional formulas are now available with "conforming" solvents.
This means that the new formulations meet the requirements and limitations of old
Rule 66, discussed earlier.
Figure 10 shows some of the types of systems that meet the requirements and the
compositions of their solvents.
The solvent-changing approach, however, has two main limitations: (a) emissions
of non-photochemically reactive solvents are still limited by Rule 442, discussed
earlier, to 396 pounds per hour and a maximum of 2, 970 pounds per day; and (b) re-
formulations generally result in higher costs. For example, a 100-percent xylene
thinner costs about 60 cents per gallon. The cost of the complying substitute formula
in Figure 10 would be 90 cents per gallon.
29
-------
Dominant resin-type in coating
Re I cost/ft2 of
a 2-mil thickness
of coating
Oxidizing alkyd
Oxidizing alkyd and melamine and/or urea
non-Oxidizing alkyd and melamine
non-Oxidizing alkyd and urea
Vinyl chloride-acetate copolymer
Acrylic-type copolymers
Styrenated alkyds (oxidizing)
Phenolic
Epoxy
Epoxy and melamine
Melamine and ethylcellulose
Polyurethane and alkyd
Sili
i cone
Silicone and alkyd
Ally I ester copolymers
Polyamide (nylon) 10 mils, flame spray
Polytetrafluoroethylene (Teflon) flame spray
Poly (chlorofluoroethylene) (Kel-F)
1.00
1 .30
1 .50
1 .35
1.50
4.00
1 .10
1.70
2.00
1 .50
1.50
1 .60
10.00
7.00
6.00
5.00
13.00
11.00
Source: Kirk Othmer, Encyclopedia of Chemical Technology, 2nd Ed., Volume 5. Interscience. 1968.
Figure 9. Relative Costs of Coatings
30
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Type of surface
coating
Enamel, air dry
Enamel, baking
Enamel, dipping
Acrylic enamel
Alkyd enamel
Primer surfacer
Primer, epoxy
Primer, zinc
chromate
Primer, vinyl zinc
chromate
Epoxy -poly amide
Varnish, baking
Lacquer, spraying
Lacquer, hot spray
Lacquer, acrylic
Vinyl, roller coat
Vinyl
Vinyl acrylic
Polyurethane
Stain
Glaze
Wash coat
Sealer
Toluene replace-
ment thinner
Xylene replacement
thinner
Weight,
Ib/gal
7.6
9. 1
9.9
8.9
8. 0
9. 4
10.5
10. 3
8. 4
10.5
6.6
7.9
8. 4
8. 4
7.7
8.9
7. 5
9. 2
7. 3
7. 8
7. 1
7. 0
6.7
6.5
Composition of surface coatings, % vol
Nonvolatile
portion
39.6
42. 8
59. 0
30. 3
47. 2
49. 0
57. 2
37. 8
34. 0
34. 7
35. 3
26. 1
16.5
38. 2
12
22. 00
15. 2
31. 7
21.6
40.9
12. 4
11.7
Hydrocarbon
Aliphatic
saturated
93.5
82. 1
58. 2
92.5
18. 0
44. 8
80. 0
17. 5
7. 0
16.4
10. 0
80. 6
91.6
40. 6
41. 2
55.5
56.5
Aromatic
6. 5
11. 7
7. 2
6. 9
7. 5
8. 9
15.9
7. 2
7.9
19. 9
1. 7
6. 8
18. 5
18. 9
19.7
14. 0
8. 4
14. 7
7. 0
17. 5
(Toluene)
7. 5
Alcohols
saturated
6. 2
30. 9
21. 8
3. 0
12. 8
26. 4
21. 3
24. 3
3. 5
10. 8
14. 7
24. 0
Ketones
80. 6
16. 5
60. 0
34. 5
97. 0
23. 2
17. 2
42. 0
43. 5
81. 1
84. 9
13. 9
0. 1
13. 7
19.1
Esters
saturated
3.7
12. 5
16. 8
28. 8
19. 2
45. 1
14. 8
26. 0
15. 1
66. 4
15. 7
18. 0
9.0
12. 0
Ethers
saturated
18. 0
7. 5
14.6
3. 0
1. 7
20. 5
56. 5
5. 3
4. 5
18. 0
Source: Air Pollution Control Manual. 2nd Ed., AP-40. EPA. May 1973.
Figure 10. Examples of Surface Coating and Added Thinner Formulas on an As-Purchased Basis
Having Conforming Solvent Systems
31
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INCREASING SOLIDS IN EXISTING COATING FORMULATIONS
An obvious step in reducing solvent emissions is to increase the solids content of
existing coating systems.
Advantages
In addition to reduced solvent emissions, particularly during application and air
drying, the benefits include:
• Reduced inventory space for drums. Drums of solvent-based coating typically
weigh 400 pounds. The following chart shows the effect of reducing solids in a
coating formula in a plant that consumes 1,200 dry pounds of coating per day.
Wet Weight
Per Drum
400
400
% Solids
30
60
Dry Weight
Per Drum
120
240
Drums Per Day
10
5
Thus, a 100-percent increase in solids made possible a 50-percent reduction in
drum storage area.
Reduced drum handling by operators. Increased solids per drum would also
reduce the number of drum changes at the coater, freeing operators for other
tasks.
Reduced energy for removing solvents. Changing from a 30- to 60-percent
solids system reduces by almost half the total solvent load that must be re-
moved. Normally, however, to achieve such a high percent of solids more
polar or higher-potency solvents must be used. These would typically have
slightly higher heats of vaporization than hydrocarbon solvents. Using typical
values, we see the effect of a change in solids on heat required to remove the
solvent:
c/c Solids by
Weight
30
60
% Solvent by
Weight
70
40
Avg. Heat of
Vaporization
of Solvent
Btu/lb .
160
200
Heat Required to
Volatilize Solvent
irom l,2001bs.
of Dry Coating
448,000
160,000
The change has resulted in a potential energy savings of almost 300,000 Btu.
Additional data on energy savings are given in Figure 11.
32
-------
Operation
Spray booth
Heat metal
Other heat losses
Oven exhaust
Tptal Btu required
for processing
Solvent Base
Thermoset Acrylic
(Baking temper-
ature - 350°F)
233,280
466,100
231,840
879,984
1,811,204
High Solids
Polyester
(Baking temper-
ature - 350°F)
233,280
466,100
231,840
94,651
1,025,871
80% Solids
Urethane
(Baking temper-
ature - 180°F)
233,280
183,110
91,080
37,184
544,654
NOTE: For these calculations it was assumed that the average yearly temperature was 52°F and that .018 Btu will
raise one cubic foot of air 1°F at 100% efficiency.
Source: Modern Paint and Coatings, March 1975.
Figure 11. Energy Requirements for Comparable Operations (Btu per hour)
• Increased potential for compliance with emission guidelines. The lower the
emissions from any part of the coating operation, the more likely that the plant
will be in compliance with emission restrictions. Care must be used in making
formula changes, however, to use solvents with emissions that are less photo-
chemically active.
• Reduced freight costs. Freight costs can easily be 2 cents per pound of gross
weight, with empty drums themselves weighing about 50 pounds. In the exam-
ple below, the freight cost for 1,200 pounds of dry coating would be reduced as
follows by a 100-percent increase in solids.
Coating
Solids
30%
60%
Pounds of
Coating Purchased
4,000
2,000
Total
Drum Wt.
400
250
Total Wt.
4,400
2,250
Freight Costs
$88
$45
Problems
There are, however, certain drawbacks to high-solids systems, including:
• Higher viscosity of the coating system. As solids are increased, so is the vis-
cosity of the formula. Typical increases in viscosities as a function of the
solid content for prepolymer coatings are given in Figure 12. Higher applica-
tion viscosity may be handled by either equipment or operational changes. An
increased coating temperature, for example, may reduce viscosities enough so
that the higher-solids system can be run on the same equipment.
33
-------
1000 -
Acrylic and Polyester Polyols
2 400 -
200 -
40
50
60 70 80
Polyol Solids, Weight Per Cent
Source: Modern Paint and Coatings, March 1975
Figure 12. Solids vs. Viscosity for Caprolactone, Acrylic, and Polyester Polyols
• Reduced storage stability. The higher the percentage of solids, the harder it
becomes to maintain a stable system. Skinning-over becomes more of a prob-
lem with higher solids, with redispersal more difficult. The tendency to thicken
or gel with time can often be counteracted by additives, but these may have del-
eterious effects on other coating properties.
• Less latitude with in-plant formula modifications. Because of the instability of
high-solids systems described above, it is usually difficult to modify them
in-plant.
Some typical formulas for high solid coatings are presented in Figure 13.
SWITCHING TO WATERBORNE SYSTEMS
Use of water-based coating systems is still a further choice of formula variations
for emission control.
Advantages
Differences between emissions from waterborne systems and solvent-based sys-
tems are shown in Figure 14. For instance, at 30-percent coating solids, a waterborne
system containing 20 percent solvent and 80 percent water would have one-quarter of
the solvent emissions of a 100-percent solvent system.
34
-------
High-Solids Coating (White) (80% Solids)
Urethane
Materials
Component 1
Multron R-2211
2-Ethyl-l,3 hexanedial2
R-966, TiO23
Modaflow, 10% in ethylglycal
acetate4
FC 430, 10% in ethylglycal acetate5
EAB 381-1/10, 10% in ethylglycal acetate6
Dibutyltin dilaurate, 10% in ethylglycal acetate7
Ethylglycal acetate
Component II
Desmodur N-1001
Total Weight Solids Weight
94.5 94.5
94.5 94.5
280.0 280.0
10.4
15.6
10.4 1.0
0.5
163.1
331.0 331.0
1,000.0 | 801.0
High-Solids Acrylic Hard Enamel
Materials
Disperse on roller mill
Titanium dioxide
Experimental Resin QR-542
Letdown
Mill paste (above)
Experimental Resin QR-542
Cymel 301
p-TSA (30% in sopropanol)
n-Butyl acetate
n-Butanol
(80% in Ektasolve EE acetate)
(80% in Ektasolve EE acetate)
Formulation Constants
Solids content
Titanium dioxide (45%)
Binder (55%)
Experimental Resin QR-542
Cymel 301 (30%)
Volatiles content
Catalyst, p-TSA (on binder)
Spray viscosity, *4 Ford cup (sec)
1. Mobay Chemical Corp.
2. Union Carbide Corp.
3. E. I. duPont de Nemours & Co.
4. Monsanto Co.
(70%)
35
Parts By Weight
60.0
40.0
100.0
24.2
22.0
0.5
14.4
11.9
173.0
Percent of Formula
77
23
0.2
5. 3M Co.
6. Eastman Chemical Products, Inc.
7. M & T Chemicals, Inc.
Source: Modern Paint and Coatings, Match 1975.
Figure 13. Examples of Modern Formulas for High-Solids Systems
35
-------
High Solids Coating (100% solvent)
_„, 100-%Solids , nn
RSE = TZZ x 1.00
Water-Borne Coating (20% solvent)
nor 100-%Solids
RSE = — x 0.20
10 20 30 40 50 60 70
Application Solids, Per Cent by Weight
Source: Modem Paint and Coatings, March 1975.
Figure 14. Relative Emissions of a Hypothetical Waterborne System Containing 20% Solvent
and of a Conventional Solvent Base System
Figure 15 further illustrates the reduction in emissions from the substitution of
waterborne coatings for conventional or high-solids systems.
Additional advantages of switching to a waterborne system include:
• Reduction of flammability levels. While many waterborne formulations include
"co-solvents," these often evaporate before heat treatment, considerably re-
ducing problems in the ovens. Much lower dilutions are required due to the
lower percentage of solvent and also to the "quenching" effects of the water
vapor.
• Increase in usable polymers. In solvent-based systems, relatively few mono-
mers or prepolymers can be used because of solubility, viscosity, and related
factors. In particular, the molecular weights are severely restricted. This
affects the ultimate properties of the coating. In waterborne coatings, the
choice of monomers and/or prepolymers is much wider.
• Higher-solids content at equivalent viscosity. In solvent polymerizations, as
the molecular weight increases so does the viscosity. Waterborne systems are
not as sensitive to viscosity from increased molecular weight. Thus, to obtain
36
-------
Paint System
1 . High-solids polyester
2. Coil-coating polyester
3 . High-solids alkyd
4. Short-oil alkyd
5. Water-reducible polyester
6. Water-reducible alkyd
7. High-solids water-
reducible conversion varnish
8. High-solids water-
reducible conversion varnish
9. High-solids water-
reducible conversion varnish
% N.V.V.*
80
51
80
34
48
29
80
73.5
67
Volume Ratio:
Total Non-Volatile/
Organic Volatile
80/20
51/49
80/20
34/66
82/18
67/33
90/10
90/10
90/10
Ml of Organic
Volatile Liberated
per Sq Ft per Mil
of Dry Film Coating
0.59
2.30
0.59
4.75
0.51
1.16
0.24
0.24
0.24
*Non-volatile by volume.
Source: Modern Paint and Coatings, March 1975.
Figure 15. Comparison of the Amount of Organic Volatile Material Contained in
High-Solids, Water-Soluble, and Conventional Paints
similar molecular weights, a solvent system must be used with a much higher
viscosity than that of a waterborne system. In addition, waterborne system
viscosities are less sensitive to solid contents than are those of solvent sys-
tems. Thus, waterborne systems permit the use of higher solids with higher
molecular weight for the same required viscosity.
Lower raw material cost. The cost of solvent coatings includes the price of
the solvent, whereas in aqueous-based coatings very little solvent is used and
the water is free. A typical example follows of raw material costs for equiva-
lent solids systems, in which the solids cost 50 cents per pound and the solvent
an average of 75 cents per gallon, or 10 cents per pound.
40% Solids ,
solvent-based
40% Solids ,
water-based
Cost of Solids
per 100 Ib.
$20.00
$20.00
Cost of
Solvent
$6.00
$1.50
Total Raw
Material Cost
$26.00
$21.50
37
-------
The cost of the solvent will be reflected in the selling price of the coating. This
was a prime factor in the significant cost increases of solvent-based coatings
during the recent petrochemical shortage.
• Ease of clean-up. Water-based systems can be readily cleaned up with water,
whereas solvent systems require solvents.
Problems
Drawbacks associated with switching to waterborne coatings include:
• Use of a comparatively untried technology. The traditional reliance on solvent-
based systems for metal coating has resulted in a lower level of interest in
waterborne systems. However, many authors claim that dry-film properties
have been developed that are equal or superior in every respect to those
achieved by conventional solvent systems.
• Higher total-system energy requirements to remove water. Water has a higher
latent heat of vaporization (1, 000 Btu/lb.)than most solvents (100-200 Btu/lb.).
Thus, it takes more heat energy to evaporate or remove a pound of water than
a pound of solvent. A comparison follows of two systems, one a 70-percent
solids solvent coating and the other a 70-percent solids aqueous coating.
Coating
Type
Solvent
Aqueous
Volatile s
Solvents
Water
Latent Heat of
Vaporization
Btu/lb.
200
1,000
Heat Required to
Volatilize l,0001bs.
Volatiles
200,000 Btu
1,000,000 Btu
As a rule of thumb, at $1.25 per 1,000 cubic feet of gas and 1,000 Btu per
cubic foot, the cost of natural gas is $1.25 per 1,000,000 Btu. Thus, evapo-
rating the solvent costs 25 cents and the water $1.25.
There may be compensating factors for the high cost of water removal, however,
in that some of the solvent that evaporates from waterborne coatings may be used for
heating requirements through burning of the oven exhaust gases. This depends on in-
dividual plant operations and will be discussed again later.
The higher energy requirement for evaporating the water is usually mitigated by
the fact that this constitutes only part of the heat loss of the oven; the exhaust gases
also carry away a portion of the heat requirement. Figure 16 compares the energy
balance in an oven curing a conventional solvent system and an equivalent waterborne
coating that has a solvent component representing 20 percent of the volatile load. In
this instance the heat requirements are quite similar, with a 10-percent edge in favor
of the waterborne system.
38
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EXHAUST RATES
Solvent Coating
17.5 gallons of solvent\ /10 .OOP cubic feet at 70°F A / 1 hour
hour / \ gallon / \60 minutes
2925 cubic feet of air at 70°F. per minute.
The exhaust rate for the solvent system is 2,925 cubic feet of air at
70°F per minute.
Waterborne Coating
A. Solvent Requirement
17.5 gallons\ /IQ.OOO cubic feet of air at 70°F \ ( 1 hour
hour / ' \ gallon / \60 minutes
= 583 cubic feet of air at 70°F . per minute.
B. Humidity Requirement
17.5 gallons\ . g) /5,OOP cubic feet of air at 70°F \ I 1 hour
hour / ' \ gallon / I 60 minutes
= 1167 cubic feet of air at 70°F. per minute.
The total exhaust requirement is 1,750 cubic feet of air at 70°F. per minute.
Solvent System
Parts and Conveyor Load
11,OOP pounds\ / 0.12Btu \
hour j ( pound °F.j (350°F.-70OF.)
370,000 Btu/hour.
Panel Loss Load
Qp = (10,000 square feet) / °-3 Btu \ (350°F .-70°F.)
sq. ft. op.
840,000 Btu/hour.
Figure 16. Heat Requirements for the Baking of Equivalent Solvent-Borne and Waterborne Coatings
39
-------
Exhaust Load
Qa =
60 minutes \
/2925 cubic feet at 70°F A /0.075 poundsX / 0.24 Btu
y minute j \ cubic foot I I pound °F.
(350°F.-70°F.)
hour )
Total 2,095,000 Btu/hour.
855,000 Btu/hour.
The total heat lost, i.e., the total heat input, is approximately 2,100,000
Btu per hour, which can be supplied by burning 2,100 cubic feet of natural
gas per hour.
Waterborne System - 350° Bake
Parts and Conveyor Load
Qm
11,000 pounds\ / 0.12 Btu
hour / \ pound °F.
(350°F.-70°F.)
370,000 Btu/hour.
Panel Loss Load
(10,000 square feet)
' 0.3 Btu
sq. ft. op.
(350°F.-70°F.)
840,000 Btu/hour.
Water Evaporation Load
Qw =
138,000 Btu/hour.
Exhaust Load
' \ / \ /
^14 gallons of water] [ 8.33 pounds\ / 1,178 Btu
hour I \ gallon / I pound °F.
Qa =
60 minutes
hour
Total:
1,750 cubic feet at 70°F.\ /0.075 pounds\
minute I \ cubic foot I
(350°F.-70°F.)
530,000 Btu/hour
1,878,000 Btu/hour
0.24 Btu
pound °F .
The total heat lost, i.e. , the total heat input, is approximately 1,900,000
Btu per hour, which can be supplied by burning 1,900 cubic feet of natural
gas per hour.
Source: Metal Finishing, December 1975.
Figure 16 (continued)
40
-------
• Rust and corrosion potential. Coating applicators, tunnels, JIK! ovens would
now be subject to water vapor, which could condense and drip down onto moving
parts. Ovens made of galvanized steel may be subject to watc-t corrosion.
• Increased treatment of metal parts before coating. Most metal parts have
films of grease and oil that must be removed to achieve proper coating films.
Solvent-based coatings have an ability to "self-clean" some of the surface if it
has not been completely treated. Water-based coatings, however, would re-
quire a completely oil-free surface, which might increase prctreatment costs.
• Slow drying at high humidity. Coating operations that depend primarily on re-
moval of volatiles during application and air drying will be slowed down on days
of high humidity and slow water evaporation.
SWITCHING TO ULTRAVIOLET OR ELECTRON BEAM CURE SYSTEMS
Ultraviolet (U.V.) or electron beam systems rely on the rapid uptake of high-
intensity energy from an external source to polymerize the low-molecular-weight com-
ponents of the coating. The materials are supplied at close to 100-percent solids so
that, except for extraneous matter, all that is applied in the first place remains in the
coating.
Advantages
Benefits of using these systems include:
• Substantial emission reduction. The systems are inherently 100-percent solids,
or 100-percent active. Emissions are only incidental and can be as little as 5
percent by weight. There is ozone from the U.V. process, but this can be min-
imized by proper controls.
• High-speed reactions. Relative typical cure times for total-sol ids coatings
would be:
Curing System Time
Electron beam 1 second or less
Ultraviolet seconds
Oven minutes
• Low operating costs. Figure 17 is a synopsis of operating-cost comparisons
for conventional, U.V. and electron beam curing. Figure 18 compares the
costs of U.V. curing vs. infrared curing. This is of particular interest, since
infrared ovens can be readily converted to U.V. units.
• Reduced floor space for coater. Ovens normally take up much of the floor
space in coating lines. A system with U.V. of electronbeam curing that re-
quires minimum oven capacity will use less floor space.
41
-------
IS3
Heat
Line Speed (fpm) 60 120 180
Beam Power
Machine Power (kW) 29004" 5800 8800
Power Costs ($/hr) 13.13° 26.25 39.38
Maintenance ($/hr) .80 1.00 1.20
Nitrogen Gas
Water
Total Costs ($/hr) 13.93 27.25 40.58
60
3.7*
100
2.00*
1.00
--
--
3.00
U.V.
120
7.4
200
4.00
2.00
--
--
6.00
Electron Beam
180 60
11.1 1.25**
300 8
6.00 .16*
3.00 2.25
1.00
.20
9.00 3.61
120
2.5
10
.20
2.50
1.50
.20
4.40
180
3.75
12
.24
2.75
2.00
.20
5.19
* Based upon an ultraviolet cure requirement of 1 j/cm .
** Based upon an electron beam dose to cure of 2.5 megarads (0.25 j/cm^ for a 1 mil coating) .
+ Based upon natural gas at 1000 BTU/cubic foot and converted directly using 1055 j/BTU.
° Based upon gas costs at $1.25/1000 cubic feet.
* Based upon power costs at 2*/kWhr.
Source "Status of Electron Beam Curing," Paper presented at the National Coil Coaters Association Meeting. Las Vegas, May 1971.
(Cost figures updated)
Figure 17. Comparative Economics of High-Speed Curing Units
-------
Oven length (ft)
Line speed (fpm)
Vehicle
Nonvolatile (%)
Film thickness (mils)
Coverage (wet) (sq ft/gal)
Coats
Cure time (sec)
Exit temp (°F)
Cool
Cost of system ($/gal)
Cost per sq ft (<:)
Per coat
Total
Power (kW)
Power per sq ft ($)
Typical U.V.
10
60
Polyester
90-100
2
700-800
1
10
100
No
5.00-6.00
0.7-0.9
0.7-0.9
100
Less than 0.1
Typical IR
90
60
Urea-alkyd
35-65
2
500
2
90
130
Yes
2.00-3.00
0.9-1.3
1.8-2.6
250
App. 0.2 (2 coats)
Source: Journal of Paint Technology, Vol. 44, No. 571, Aug. 1972.
Figure 18. Comparative Costs of U.V. Curing and Infrared Curing
Problems
The disadvantages of switching to U.V. or electron beam curable coatings systems
are:
• High formula costs. As can be seen in Figure 18, formulas based on the types
of polymers that can be cured by U.V. or electron beam cost several times as
much as conventional coatings, necessitating tight control on overcoating and
waste.
• Limited selection of polymers. Since this is a relatively new technology, the
range of polymers available is still limited, although some can coatings, var-
nishes, and inks have been developed.
43
-------
• Special precautions for high intensity energy sources. U.V. and electron beam
energy sources can cause injuries to workers if not carefully shielded and
operated.
• High costs of coating and curing equipment. Initial capital expenditures for
coaters, curing chambers, and protective shielding tend to be higher than
equipment for conventional coatings.
SWITCHING TO 100-PERCENT SOLIDS COATINGS
Total-solids systems represent an entirely different technology and for most ap-
plications require new equipment that is generally not compatible with existing lines.
Because of the potential advantages, however, the automotive industry has begun using
some of these coatings for auto bodies. Further, chain-link fencing is processed with
a "green" coating, and many houseware items have protective plastic coatings.
Advantages
Benefits of using 100-percent solids coating systems include:
• Freedom from emissions. There is no solvent vapor generated in the curing
process for total-solids coatings. Emissions are therefore negligible and are
limited to solid particles that can be trapped by relatively cheap systems like
dust collectors.
* Reduced energy consumption. Since the coating is 100-percent solids, no heat
is required to volatilize solvent or water. The only heat needed for thermo-
plastic coatings is that necessary to melt or flux the material so that it will
bond to the surfaces.
Heats of fusion or melting tend to be lower than heats of vaporization, so that the
net heat required per 100 pounds of dry coating would be less than that for either the
high-solids or aqueous systems. Additional heat will be needed to cure the coating if
it is a thermosetting type; however, since no solvent or water has to be removed, the
total heat will still be lower than for an equivalent waterborne or high-solids solvent
system.
Problems
Disadvantages of 100-percent solids coating systems are:
• Higher costs. On a relatively equivalent basis, solvent-based paints were ap-
proximately 1-1.3 cents/ft2/mil of thickness, whereas fluidized-bed powder
coatings were 1.6 - 4.1 cents/ft2/mil, depending on the system.
• Limited selection of systems. Only certain polymers are available in a form
that will flux and fuse (polyamides, polyesters, and some epoxies), limiting
total-solids coating formulations.
44
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• Variable adhesion. Adhesion, a direct function of the fusion process, may be
adversely affected by any irregularity in temperature in either a 100-percent
solids coating or the surface to be coated.
• Incompatibility with existing coating lines. As mentioned, special new equip-
ment is required for application and curing of 100-percent solids systems.
• Difficulty in applying uniform thin coatings. The total-solids coating technique
lends itself to thicker coatings. Applications under Imil are difficult; 3 or
more mils is more typical.
• Color changes in-process. In 100-percent solids coating lines, large amounts
of colored particles must be moved and cleaned up before each color change or
the next batch of articles may have off-specification colors or shades.
EMISSION CONTROL THROUGH PROCESS CHANGES
Operating changes that a plant can consider in setting up its emission control pro-
gram include:
• Controlling emissions by incineration;
• Controlling emissions by adsorption;
• Improving spraying efficiency;
• Improving dip coating, flow coating, and coil coating efficiency;
• Purchasing prefinished roll stock;
• Increasing vapor concentration; and
• Educating plant personnel for process changes.
The first options, controlling emissions by incineration and adsorption, will be
covered in Part B of this manual, which deals with treatment of hydrocarbon emis-
sions and heat recovery. Discussion of the remaining process changes follows.
IMPROVING SPRAYING EFFICIENCY
The most commonly used air-spraying method, as explained earlier, is the most
inefficient coating method. Overspray (and thus emissions) can often be reduced by
ganging spray nozzles of different spray patterns or by rotating the article to be
sprayed. Prefinishing the article so only a touch-up is required may also cut spraying
losses.
Other techniques for improving efficiency include minimizing manual spraying and
color changeovers through production control.
45
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The effect of inefficient spraying on emissions is obvious. More air is required
to maintain the TLV, necessitating more fan capacity. If emissions have to be con-
trolled, added cfm are very expensive, as is shown below:
Effect of Over spray Reduction
Coating Formulation
20/80 Coating system
TLV
200ppm
200ppm
Over spray
50%
10%
scfm
1,400
1,080
In this case, the savings in fuel for the afterburner would amount to about $4,000 per
year, which would be in addition to a savings of about 27 percent on the cost of the coat-
ing system used.
A gross measurement of overspray can be obtained by a material balance between
the coating actually used and the coating that is on the articles after spraying.
IMPROVING DIP COATING, FLOW COATING, AND COIL COATING EFFICIENCY
As in all coating operations, control of coating weight or thickness is of primary
importance. There are many devices that can be installed on production lines for
sampling on a random basis and for weighing the article if it has, for example, been
dip coated and can be weighed. Off-weights will trigger either a manual or automatic
response to correct the situation. In the case of dip coating, this corrective response
might be shorter immersion time, reduced immersion depth, or increased air blow-off
pressure.
Beta ray, gamma ray, and x-ray devices have been used in many areas of industry
for determining coating thickness on moving webs. Their use in monitoring high-speed
coil-coating applications should be considered.
PURCHASING PREFINISHED ROLL STOCK
Some items lend themselves to prefinishing and use of raw materials that come in
coil/machinable form. For example, license plate stock is prime-coated at one loca-
tion and stamped and painted at another. This not only places the prefinishing step in
a more efficient setting, but also shifts some of the solvent emission load. Since a
final product is still the responsibility of the ultimate finisher, however, precise con-
trol must be maintained over the prefinisher.
INCREASING VAPOR CONCENTRATION
The cost of moving and heating air is proportional to the amount of air being
moved. There is, therefore, a considerable operating-cost advantage in having vapors
as concentrated as possible.
46
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In our initial discussion of basic factors that affect emissions, we pointed out that
to maintain a safe TLV, 30 times more air must be supplied to the spray booths or
air-drying tunnels than is strictly necessary for a normal rate of evaporation. If
booth and tunnel areas can be kept free of operating personnel, however, the TLV con-
centration requirements can be replaced by the much less demanding 25-percent LEL.
Further, operators can continue to work in the areas by using protective devices.
Substitution of automatic spraying wherever possible will eliminate the need to
maintain TLV, with important economic advantages. For instance, without TLV stric-
tures the ventilation requirements can be reduced by a factor of about 10, lessening
energy needed to move the air or to control its temperature. Moreover, emission
control becomes much cheaper because (a) equipment size is drastically reduced, with
savings of 40 percent; and (b) fuel costs are also greatly reduced because less air has
to be heated and much less fuel is required per scfm.
An important consequence of increasing vapor concentrations is that all equipment
conveying wet parts must be enclosed. However, the economic advantages of increased
concentrations may pay for the substantial modifications that enclosure requires.
Vapor concentrations cannot be raised beyond safe limits or the limits placed on
recovery incineration equipment. For example, if emissions are controlled by com-
bustion with either an afterburner or catalytic converter, there is no point going above
40 percent LEL; because of the considerable heat value of most solvent vapors, partic-
ularly hydrocarbons, severe overheating and equipment damage may result from excess
vapor combustion. Indeed, this is reported to be one of the most frequent problems
with afterburners, especially with catalytic units.
EDUCATING PLANT PERSONNEL FOR PROCESS CHANGES
The main problem in switching to a more efficient application method (without sig-
nificant change in system formula) may be human resistance to change. This is par-
ticularly true where hand operations are replaced by automated methods. A change
that may even temporarily affect quality or production rate may be resisted by super-
visory personnel who pride themselves on high-efficiency/low-downtime operations.
Therefore, any test of new equipment (for instance, airless or electrostatic) should be
closely supervised by management-level staff.
A second problem is that a new process often involves new materials with higher
costs per pound. Economic advantage can be achieved only if the product is used at the
prescribed rate, restricting the operating personnel's latitude in applying the coating.
Previously, if the coating was within 10 - 20 percent of desired weight or thickness,
there was little cost effect. Higher costs per pound necessitate more precise control.
Management must make it clear to employees that the changes are in everyone's
best interest.
47
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SUMMARY
In Part A of this volume, we have covered the basic terms connected with pollution
control, the properties of solvents that can cause pollution, and the ways that materials
and processes influence the amount of emissions.
We have seen that the factors most affecting emissions are:
• Total volume or weight of coating used;
• Efficiency of application; and
• Composition of the formulas used.
Many routine opportunities for pollution reduction will become evident in a simple
but thorough survey of the plant. Some possibilities may be beyond one department's
direct control, but cooperative effort with other sections may enable considerable re-
ductions in pollution and costs.
PRODUCT DESIGN CONSIDERATIONS
Certain variables in design should be studied as possible aids to pollution control.
Managers should consider:
• Choosing material that will serve the intended use without painting, for instance,
anodized aluminum, plastic, or plated components;
• Standardizing and reducing the number of colors to minimize solvent needed for
clea,n-up between color changes and to reduce inventory;
• Tightening specifications on coating thicknesses or number of coats required;
and
• Eliminating pockets, rough coatings, or other features that require large
amounts of paint for adequate coverage.
FABRICATION CONSIDERATIONS
Manufacturing variables that may aid in pollution control include:
• Using preconted stock and limiting painting operations to touch-up of damage
occurring during fabrication;
• Buying primed components and applying only a top coat;
48
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• Increasing protection, and saving paint, by using conversion coatings such as
anodizing and phosphatizing;
• Assembling first and saving painting for the final step, to avoid two paint appli-
cations; and
• Fabricating the articles and then subcontracting the coating operation to a fin-
isher who already has emission controls in place.
PROCESS CONSIDERATIONS
We have discussed in some detail the process changes that can be made to reduce
emission levels in coating operations. Those that will have a high impact on lowering
emissions at the source include:
• Replacing manually operated, air-atomized spray methods with, preferably, a
combination of airless and electrostatic spraying, to reduce over spray and help
reduce ventilation needs;
• Converting to formulas with as high a solids content as possible;
• Switching wherever possible to waterborne coating;
• Reducing excessive ventilation;
• Using powder coating and U.V. curing, where feasible, when new facilities are
installed.
If control devices prove necessary even after all possible design, manufacturing,
and process changes have been made, plant management should carefully examine the
total air balances in the facility and should study all unavoidable sources of emission,
with a view to increasing concentrations in waste streams.
The end-of-line treatment of these waste streams will be the subject of Part B,
which follows.
49
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Part B
TREATMENT OF HYDROCARBON
EMISSIONS AND HEAT RECOVERY
50
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CHAPTER I
DISPOSAL OF SOLVENT VAPORS
Most plants have hydrocarbon emissions that cannot be eliminated by the source
control methods discussed in Part A. There are a number of techniques for treating
these emissions, of which the most widely used are based on combustion that breaks
down organic pollutants into other, nonpollutant substances.
COMBUSTION
Combustion of organic compounds is a widely used technique for air pollution
emissions control. At a high enough temperature, carbon and hydrogen will combine
with oxygen to produce carbon dioxide and water. Although there is some concern
over accumulation of carbon dioxide in the atmosphere, both CO, and water will prob-
ably remain classified as nonpollutants for the foreseeable future.
Elements other than carbon and hydrogen that may be present in the organic com-
pound will also be released (though not necessarily in oxygenated form) in the combus-
tion process. Halogenated hydrocarbons like chlorine and fluorine are generally con-
verted to the acids, though in certain cases phosgene may result. Sulfur is burned
to sulfur dioxide, while nitrogen is converted to nitric oxide.
Combustion is used for control of odorous sulfur and nitrogen compounds where
the amounts of SO2 or NO formed are too small to cause significant air pollution.
However, halogenated compounds are not normally burned, because of the extremely
corrosive and hazardous nature of the gases formed. Even trace quantities of HC1 or
HF would force the use of exotic and expensive corrosion-resistant materials in the
control equipment. Greater-than-trace quantities would require additional controls
for the removal of acid gases. Thus, combustion for the control of halogenated hydro-
carbons is impractical.
In a recent review of solvent metal cleaning practices in industry,* Dow Chemical
Company found that halogenated hydrocarbons are used almost exclusively in vapor de-
greasing and in about half of the cold degreasing operations. From a practical stand-
point, therefore, solvent combustion as an air pollution control technique is limited
largely to the metal coating industry.
*"Study to Support New Source Performance Standards for Solvent Metal Cleaning Operations." Report to EPA by
The Dow Chemical Company, June 1976.
51
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Combustion of solvent vapors can be accomplished in one of three ways:
• Direct-flame incineration
• Catalytic incineration
• Process boilers
DIRECT-FLAME INCINERATION
Direct-flame or thermal incineration involves raising the waste gas temperature
and sustaining it long enough for any hydrocarbons present to combine with available
oxygen. A direct-flame incinerator usually consists of a burner fueled with auxiliary
fuel and a mixing chamber. The efficiency of the unit depends on the temperature and
residence-time characteristics of the unit and, to a lesser degree, the solvent burned
and the design details.
Eighty-five percent combustion of most contaminants is easily obtainable at tem-
peratures of 1200°F - 1400°F and 95 percent at approximately 1500°F. For direct-
flame units without heat recovery, the principal expense is fuel. The addition of heat-
recovery equipment will increase capital costs but reduce those for fuel.
Gas Conditioning
Any non-combustible material, such as particulate matter in the waste gas, will
simply pass through the incinerator at normal temperatures. Since the gas velocities
are generally lower in the combustion chamber than in the incoming ductwork, the
combustion chamber will act as a settling chamber and dust will tend to accumulate
there. This does not normally affect the performance of the unit unless the buildup
significantly reduces the combustion chamber volume or alters the flow pattern. Where
the incinerator exhaust is circulated back into the oven, the presence of particulate
matter may affect the quality of the coating.
In most metal coating operations, the carryover of particulate matter is insignifi-
cant and no prior conditioning or precleaning is necessary. Where large amounts of
paint are likely in the exhaust gas, a dry-type collector is preferred to avoid cooling
of the gases and increased incinerator fuel consumption.
Combustion Conditions
To achieve efficient combustion of hydrocarbons to carbon dioxide and water, the
solvent must be mixed with sufficient oxygen held at a uniform temperature of between
1200°F and 1500°F for 0.3 - 0.5 seconds. Time and temperature are interrelated, so
that a relatively short contact period and high temperature can produce an efficiency
(i.e., degree of pollutant destruction) equivalent to a time/temperature unit with long
contact and low temperature. This effect is illustrated in Figure 1. For normal
straight-chain solvents, operating temperatures of from 1200°F to 1300°F at a resi-
dence time of 0.3 - 0.5 seconds are generally used to achieve greater than 90 percent
control. Methane, cellosolve, or benzene-based compounds, however, may require a
temperature of 1400°F - 1500°F at conventional contact periods of 0.3 - 0.5 seconds.
52
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en
oo
<£ 40
600
800
1000
1200 1400
Increasing Temperature
1600
1800
2000
Source: Afterburner Systems Study, Shell Development Company, 1972.
Figure 1. Coupled Effects of Temperature and Time on Rate of Pollutant Oxidation
-------
In cases where carbon monoxide formation in the incinerator is deducted from the
unit's efficiency, such as under Rule 66 of the Southern California Air Pollution Con-
trol District referred to earlier, significantly higher time/temperature units are re-
quired to achieve a given efficiency. This principle is illustrated in Figure 2. The
combustion of organic carbon to carbon dioxide is a two-stage reaction: the first stage
of oxidation to CO involves a relatively high-heat release and proceeds rapidly. The
second stage, further oxidation to CO2, gives off less heat and is therefore an inher-
ently slower reaction.
The zone of combustion consists of a region of rising temperature followed by a
dwell region with an essentially constant temperature. The design residence time of
0.3 or more seconds should apply to the reaction zone only, with additional volume
provided for initial combustion and mixing. Insufficient combustion chamber volume
is probably the most significant design flaw in units that fail to meet performance
expectations.
100
90
•2 80
Hydrocarbon + CO,
per LAAPCD, Rule 66
70
60
50L
1200
1300
1400
1500
Temperature. °F
Source: Afterburner Systems Study, Shell Development Company, 1972.
Figure 2, Typical Effect of Operating Temperature on Effectiveness
of Thermal Afterburner for Destruction of Hydrocarbons and CO
54
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Thermal design must take into account the following factors:
• Efficiency increases with operating temperature.
• Efficiency increases with detention time of up to approximately one second.
• Efficiency increases with initial hydrocarbon concentration.
• Efficiency decreases if the waste gas is preheated to a point approaching the
combustion temperature.
• Efficiency increases with the degree of contact between the flame and the sol-
vent vapors.
• Poor mixing yields low efficiency even if the temperature and residence time
are sufficient.
• Carbon monoxide removal requires a minimum temperature of 1300°F regard-
less of retention time.
Process Design Principles
The process design of a thermal incinerator involves selecting the general charac-
teristics for the unit, establishing design values for temperatures and gas volumes,
and determining the fuel-firing rate and combustion chamber volume. Once the proc-
ess has been fully described, the physical facilities for meeting process requirements
can be determined.
The information required for the process design calculations is:
• Inlet gas flow rate, scfm;
• Inlet gas temperature, °F; and
• Solvent type and vapor concentration range, % or ppm.
"Where a heat exchanger is used to preheat the gas, the temperature at the inciner-
ator inlet will be greater than the temperature at which the waste gas leaves the proc-
ess. Heat exchanger design considerations are further described below under Heat
Recovery.
The desired gas temperature at the incinerator must be specified. Frequently,
air pollution regulations require the gas temperature to be above a certain minimum.
This may vary from about 1250°F for easily oxidized solvents to 1500° F for resistant
vapors. Where carbon monoxide formation must be prevented, a minimum design
temperature of 1400°F is recommended. The desired gas temperature at the inciner-
ator should be slightly in excess of the required minimum.
The residence times of 0.3 - 0.5 seconds mentioned earlier should be considered
as minimum values for systems burning hydrocarbon solvents without significant ob-
jectionable impurities. Many units are operating satisfactorily at 0.3 seconds resi-
dence time, but only where extremely good mixing is achieved. For carbon monoxide
removal, the higher residence time of 0.5 seconds should generally be used.
55
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Prior to design of the incinerator, a burner type and fuel should be selected that
will be compatible with the source of oxygen for combustion, to the extent that this is
known. If the contaminated air stream will be used to provide the oxygen, the size of
the incinerator and the heat requirements will be lower, since it will not be necessary
to accommodate outside air in the system. Burner types will be discussed presently.
Natural gas and propane-fired units use contaminated air almost exclusively. Oil-
fired burners may be set up to use contaminated air, but frequently use outside air to
avoid fouling of the primary air blower and burner gun.
Hydrocarbon solvents used in metal coating have a high fuel value (Btu per pound
of solvent) and will contribute to the heat required for incineration. At concentrations
of 100-200ppm, the fuel saved will be almost negligible, but the savings will be signif-
icant beyond 25-percent LEL concentrations. Heat available from the solvent is nor-
mally included in the heat balance.
Fuel Requirements
The first step in the design of a thermal incinerator is to determine the amount of
fuel required to heat the waste gas stream to the design temperature. The amount of
fuel will depend on the flow rate, composition, and temperature of the incoming waste
gas, the type of fuel, and whether the oxygen required will be derived from the waste
gas or from external air. Any heat value from the solvent vapors in the waste gas
stream will reduce the fuel needs.
When a fuel is burned, carbon and hydrogen in the fuel combine with air to produce
carbon dioxide and water. The heat energy released raises this carbon dioxide and
water to a very high temperature. When air is used as an oxygen source, the nitrogen
present must also be heated, lowering the temperature in the gas mixture (approxi-
mately 3400°F). If the combustion products are further mixed with a waste gas stream,
the temperature of the resultant mixture will be still lower. In a normal design situa-
tion, the final desired temperature of the gas mixture is known and the problem be-
comes one of finding the proper fuel-addition rate.
The potential heat energy released by various organic materials burned at 60°F is
termed the gross or higher heat value. Since water is a combustion product of most
fuels, the energy available to heat combustion products must be reduced by the heat of
vaporization of the water formed. The resultant heat is termed the net or lower heat
value. Gross and net heat values, along with combustion air requirements for a num-
ber of fuels used in incineration, are shown in Table I. In heating waste gas streams,
the net heat released is distributed among the combustion products and the waste gases.
Since the ratio of combustion air to fuel is known, calculations are simplified by work-
ing with the concept of available heat. This is excess heat remaining for other pur-
poses after the combustion products have been heated to a specified temperature. As
may be seen in Figure 3, the available heat is reduced as the design temperature in-
creases, since more heat is consumed in heating the combustion products. If excess
air is included with the combustion products, the available heat for other purposes is
reduced still further, as is shown in Figure 4. It should be noted that Figure 4 is ap-
proximate, in that it is assumed that the ratio of combustion air to heat content is con-
stant for all fuels.
56
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Table I
Combustion Constants
No . Substance
1 . Carbon*
2. Hydrogen
3. Oxygen
4. Nitrogen (atm)
5. Carbon monoxide
6. Carbon dioxide
Paraffin series
7. Methane
8. Ethane
9. Propane
1 0. n-Bufane
1 1 . Isobutane
12 . n-Pentane
13. Isopentane
14 . Neopentane
15 . n-Hexane
Olefin series
16. Ethylene
17 . Propy lene
18. n-Butene
1 9. Isobutene
20. n-Pentene
Aromatic series
21. Benzene
22. Toluene
23. Xylene
Miscellaneous gases
24. Acetylene
25. Naphthalene
26. Methyl alcohol
27. Ethyl alcohol
28. Ammonia
29. Sulfur*
30. Hydrogen sulfide
T 1 ^i 1 1 fpr r\ invi r\f*
32! Water Vapor
33. Air
Formula
C
H2
02
N2
CO
C02
CH4
C2H6
C3H8
£4^10
C«H10
C6H12
C6H,2
^-6^12
C6H1
-------
Commercial butane
3210 Btu/cu.ft.
Commercial propane
2558 Btu/cu.ft.
600
900
1200 1500 1800 2100
Flue gas exit temperature °F
2400 2700 3000
Source: Control of Gaseous Emissions. EPA Air Pollution Training Institute, 1973.
Figure 3. Available Heats for Some Typical Fuels (Referred to 60°F)
58
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This chart is only applicable
to cases in which there is no
unburned fuel in the products
of combustion.
The average temperatuie of the
hot mixture just beyond the end
of the flame may be read at the
point where the appropriate %
excess air curve intersects the
zero available heat line.
1200 1600 2000
Flue gas temperature F
2400 2800 3200
Source: Control of Gaseous Pollutants, EPA Air Pollution Training Institute, 1973.
Figure 4. Generalized Available Heat Chart for All Fuels at Various Flue Gas Temperatures and
Various Excess Combustion Air (Referred to 60°F)
When the available heat of combustion is distributed in the waste gas, its temper-
ature will rise. However, this rise is not uniform throughout the combustion range.
For this reason, it is common practice to work in terms of enthalpy (heat content) of
the gases at various temperatures. Enthalpies of common gases are shown in Table II.
As this table shows, raising the temperature of one standard cubic foot of CO^ from
200°F to 1200°F requires 33.55 - 3.39, or 30.16 Btu.
When the combustion air is drawn from outside the waste gas stream, calculations
of fuel requirements are relatively simple; the use of waste gas for combustion re-
quires further data on combustion air requirements. Data needed for computing natu-
ral gas or propane requirements are given in Table III; data for oil are presented in
Table IV.
Sample computations follow for determining fuel requirements for various design
conditions.
Computations for Gas Requirements
Example 1—Given: 3000 acfm of air containing 500ppm of toluene. The air temper-
ature is 300°F.
Find: The amount of natural gas required to heat the gas stream to
1400°F, assuming that combustion air is drawn from the gas
stream and ignoring the fuel value of the solvent.
59
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Since the combustion air is to be drawn from the waste gas stream, it will be neces-
sary to write an equation balancing heat input and consumption in terms of an unknown
gas quantity:
1. Heat Input
= (Available heat at 1400°F at 0% excess air) x G +
(Credit for preheat of combustion air from 60°F to 300°F)
where G = scfm of natural gas required.
• From Table IE,
- Available heat at 1400°F = 668.6 Btu/scf gas
- Amount of combustion air required at 0% excess ai~
= 10.36 scf/scf gas
• From Table II, for air, enthalpy difference (300°F - 60°F)
= 4.42 Btu/scf air
2. Heat Consumption
= (scfm waste gas - scfm required for combustion) x
enthalpy difference (1400°F - 300°F)
• scfm waste gas = 300 acfm x = 2053 scfm
4oO ~r~ oUO
• scfm required for combustion = 10.36 x G scfm
• From Table II, for Air, enthalpy difference (1400°F - 300°F)
= 26.13 - 4.42
- 21.71 Btu/scf
• Thus, heat consumption
= (2053 - 10.36 x G) x 21.71 Btu/min.
3. Heat Balance: Heat Input = Heat Consumption
• Thus, 668 xG +4.42 x 10.36 xG - (2053 - 10.36xG) x 21.71
• Solving for G, G = 47.5 scfm natural gas
60
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Example 2—Using the data from Example 1, compute the gas consumption considering
the heat available from the combustion of toluene.
Toluene combustion will enter the heat balance by providing a heat input and by reducing
the unburned air that must be heated to the design temperature.
1. Toluene Flow Rate
= 2053 scfm waste gas x —7 = 1. 027 scfm
2. Gross Heat for Toluene
• From Table I, for toluene, gross heat
= 4484 Btu/scf x 1.027 scfm
= 4605 Btu/min
3. Available Heat from Toluene
• Using Figure 4, at a flue gas temperature of 1400°F and at 0% excess air,
available heat from toluene
= 0.61 x Gross Heat
= 0.61 x 4605 Btu/min
4. Credit for Preheat
• From Table I, combustion air required
= 42.88 scf air/scf toluene
• Total combustion air
= 1.027 x 42.88 scfm air
= 44.04 scfm air
• Credit for preheat
- 44.04 scfx 4.42 BtU
scfm
= 44.04 x 4.42 Btu/min
61
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5. Total Available Heat From Toluene
= available heat + credit for preheat
=- 0.61 x 4605+44.04 x 4.42 Btu/min
- 3003.7 Btu/min
6. Heat Balance: Heat Input - Heat Consumption
668 x G + 4.42 x 10.36 x G + 3003.7
= (2053 - 10.36 xG - 44.04) x 21.71
• Solving for G, G = 43.3 scfm natural gas
Comparing the two examples, it may be seen that the natural gas savings is 4.2
scfm, or 8. 8% if the heat value of the solvent is considered.
Computations for Oil Requirements
To illustrate the effects of oil firing and the use of external combustion air on the
calculations, the situation in Example 2 may be reworked for oil firing.
Example 3—Determine the quantity of #2 fuel oil (PS 100, Table IV) required to incin-
erate 3000 scfm of air conditioning 500ppm of toluene, where the air temperature is
SOOT. The oil burner is supplied with 120% theoretical air taken from outside the
waste gas stream.
1. 120% theoretical air = 20% excess air
From Figure 4, the available heat at 1400°F and 20% excess air is approxi-
mately 55% of the gross heat value of fuel oil, which is 136,000 Btu/gal. from
Table IV.
2. Heat Input
= the sum of the available heats of oil and toluene plus credit for preheat
combustion air:
= 0.55 x 136,000 x Q + 3,003.7
+ 197.3 ^- x 6.83 —. x 4.42 ^ x Q
lb gal ft-3
where Q =- quantity of oil burned, gallons/minute
62
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= 74,800 x Q + 3,007 + 5,596 x Q
= 80,756 x Q + 3,003.7 Btu/min
3. Heat Consumption is the waste gas flow less the air consumed in the combus-
tion of toluene
= (scfm waste gas - scfm required for combustion of toluene)
x enthalpy difference (1400°F - 300°F)
• Thus, heat consumption
TJ-j-,,
= (2053 scfm - 44.04 scfm) x 21.71 —7-
= 43,615 Btu/min
4. Heat Balance: Heat Input = Heat Consumption
• Thus
80,756 Q + 3,003.7 = 43,615
• where
Q = 0.5 gallons/minute
A comparison of gross heat inputs for Examples 2 and 3 shows: •
gross heat input (Example 2) = 43.3 scfm x 1100 Btu/scf
= 47,630 Btu/min
gross heat input (Example 3) = 0.5 gal/min x 136,000 Btu/gal
= 68,000 Btu/min
Avoiding the use of outside air for fuel combustion (as in Example 2) results in a
significant savings of heat input and thus of operating costs.
Combustion Chamber Size
The size of the combustion chamber will be determined by both the volumetric flow
rate of the waste gas stream and combustion products at the design temperature and
the design retention time. Since the combustion chamber should be considered as only
that zone in which the design combustion temperature is attained, some burner types
may necessitate a mixing zone before the combustion zone. Calculations follow for the
combustion chamber volume.
63
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Example 4— For the conditions in Example 2 , find the combustion chamber volume re-
quired for combustion at 1400° F using natural gas fuel with internal combustion air;
the desired retention time is 0.50 seconds.
1. From Example 2, waste gas required for the combustion of natural gas
= 10.36 scf/scf x 43.3 scfm
= 448.6 scfm
Combustion products from natural gas (from Table III)
= 11 . 453 scf/scf x 43 . 3 scfm
= 495.9 scfm
2. From Example 2, waste gas required for the combustion of toluene
= 42. 88 scf/scf x 1. 027 scfm
= 44. 04 scfm
Combustion products from toluene (from Table I)
= (7.0+3.0+33.88) scf/scf x 1 . 027 scfm
= 46.09 scfm
3. Flow through the combustion chamber
= (2053 - 448.6 - 44.04) + (495.9 + 46.09)
= 2102 scfm
= 7520 acfm at 1400 °F
4. Volume of combustion chamber needed for 0.5 second retention time
= 7520 acfm x 0.5 second x —
60 second
= 62.67 cubic feet
64
-------
Table II
Enthalpies of Gases Expressed in Btu/scf of Gas, Reference 60°F
°F
60
77
100
200
300
400
500
600
700
800
900
1,000
1, 100
1,200
1,300
1,400
1, 500
1, 600
1,700
1,800
1,900
2, 000
2, 100
2,200
2, 300
2,400
2, 500
3,000
3, 500
4, 000
4,500
5, 000
5,500
6,000
6,500
N2
-
0. 31
0. 74
2. 58
4. 42
6.27
8. 14
10. 02
11. 93
13. 85
15.80
17. 77
19.78
21. 79
23.84
25. 90
27. 98
30. 10
32.21
34. 34
36. 48
38.65
40. 84
43. 00
45. 24
47. 46
49. 67
60. 91
72. 31
83. 79
95. 37
107.04
118. 78
132. 54
142. 37
°2
-
0. 31
0. 74
2.61
4. 50
6. 43
8. 40
10. 40
12.43
14. 49
16. 59
18. 71
20. 85
23. 02
25.20
27. 40
29. 62
31.85
34. 10
36. 34
38. 61
40. 90
43. 17
45. 47
47. 79
50. 11
52. 43
64. 18
76. 13
88. 29
100. 64
113.20
125. 89
139. 74
151. 72
Air
-
0. 32
0. 74
2.58
4. 42
6.29
8. 17
10.07
12. 00
13. 95
15.92
17. 92
19.94
21.98
24. 05
26. 13
28.24
30.38
32.50
34. 66
36.82
38.99
41. 18
43. 39
45. 61
47.83
50.07
61. 39
72. 87
84. 42
96. 11
107. 91
119.78
131.73
143. 76
H2
-
0. 31
0. 73
2. 55
4. 40
6. 24
8. 09
9.89
1 1. 77
13.61
15. 47
17. 36
19. 20
21. 08
22. 95
24.87
26. 80
28. 70
30.62
32. 52
34.45
36. 43
38.49
40. 57
42.66
44. 71
46. 82
57. 22
68. 14
79. 38
90. 68
102. 42
114. 21
126. 16
138. 35
CO
-
0. 32
0. 74
2. 58
4. 43
6.29
8. 18
10. 08
12. 01
13. 96
15. 94
17. 94
19.97
22. 02
24. 10
26. 19
28. 31
30. 44
32. 58
34. 74
36. 93
39. 12
41. 31
43. 53
45. 74
47. 99
50. 23
61. 55
73. 00
84. 56
96. 21
107. 93
119. 70
131. 52
143. 37
CO2
-
0. 39
0. 94
3. 39
5. 98
8. 69
11. 52
14. 44
17. 45
20. 54
23. 70
26. 92
30. 21
33. 55
36. 93
40. 36
43. 85
47. 35
50. 89
54. 48
58. 07
61. 71
65. 35
69. 02
72. 71
76. 43
80. 15
98. 96
1 18. 15
137. 62
157. 20
176. 93
196. 77
216. 77
236.88
H20a
-
0. 36
0. 85
2.98
5. 14
7. 33
9.52
11.81
14. 11
16. 45
18. 84
21. 27
23. 74
26. 26
28.82
31. 42
34. 08
36. 77
39. 49
42. 26
45. 06
47. 91
50. 78
53. 68
56. 64
59.59
62. 60
77. 98
93. 92
110. 28
126. 96
143. 92
161. 07
178. 41
195.82
Enthalpies are for a gaseous system, and do not include latent heat of vaporization.
Lv = 1, 059. 9 Btu/lb or 50. 34 Btu/scf of H2O vapor at 60°F and 14. 696 psia.
Source: Air Pollution Engineering Manual, EPA AP-40, 2nd Ed., 1973.
65
-------
Table III
Combustion Characteristics of Natural Gas
Average analysis
CO,
N2
°2
CH
C2H6
C,HR
"C4H10
n"C4H10
cs+
c +
i, volume %a
0
5. 15
0
81. 11
9.665
3. 505
0. 19
0.24
0.09
0. 05
100.00
Average gross heat, 1, 100 Btu/ft
Air required for combustion
Theoretical - 10.360 ft /ft gas
20% excess air - 12. 432 ft'/ft3 gas
Products of combustion/ft of gas
Theoretical air
2QTo excess air
Vol
CO2 1. 134 ftj
2. 083
8.236
Wt
Vol
0. 132 Ib
0.099
0. 609
Total 11.453 ft^
Available heat, Btu/ft
0. 840 Ib
1. 134 ftj
2.083
9.873
0.435
13.525 ft3
Wt
0. 132 Ib
0. 099
0.731
0. 037
0. 999 Ib
as,a based on latent heat of vaporization of water at 60°F
Available heat, Btu,
i emp, r rtvauaoie neat, DLU, tU7o excess air
with theoretical air
j
(
t
t
t
t
t
t
i,
i.
2,
2,
2,
2,
2,
2,
3,
3,
100
150
200
250
300
350
400
450
500
550
600
700
800
900
000
100
200
300
400
500
600
700
800
900
000
100
200
300
400
500
000
500
988.
976.
963.
952.
941.
928.
917.
906.
894.
882.
870.
846.
820.
797.
772.
747.
721.
693.
668.
642.
614.
589.
562.
534.
507.
478.
450.
421.
393.
364.
219.
70.
6
1
7
1
0
8
8
2
6
7
9
2
7
7
6
2
3
0
6
7
6
8
3
8
5
7
7
9
0
6
1
4
992.
97i.
958.
949.
932.
917.
905.
891.
878.
864.
850.
821.
792.
765.
736.
706.
676.
643.
615.
584.
552.
523.
491.
459.
428.
394.
362.
329.
295.
262.
94.
--
2
0
5
9
0
8
1
5
0
1
4
8
3
3
2
6
5
6
4
5
9
7
7
9
2
9
5
1
6
6
2
aAverage of two samples analyzed by Southern Calif. Gas Co. , 1956.
Source: Air Pollution Engineering Manual, EPA AP-40, 2nd Ed., 1973.
66
-------
OS
Table IV
Combustion Data Based on 1 Pound of Fuel Oila>
Constituent
Common Name
Density, Ibs./gal.
(40% sat'd at 60°F)
Flue gas
constitu-
ents with
theoret-
ical air
CO
SO*
N2
fLO formed
H \ Q (fuel)
H^O (air]
Total
Amount of
flue gas
with %
excess
air as
indicated
- o
7.5
10
12.5
15
17.5
20
30
40
50
75
100
SCL % by vol. and wt.
with theoretical air
Approximate Btu/gal.
PS No. 100
Kerosine/
Distillate
6.83
ft3
197.3
26.73
0.002
154.8
28.76
-
1.367
211.659
211.7
226.5
231.4
236.4
241.3
246.2
251.2
270.9
290.6
310.4
359.7
409.0
0.0011
Ib
15.04
3.104
0.004
11.44
1.368
-
0.0662
15.9786
15.98
17.11
17.48
17.86
18.24
18.61
18.99
20.49
22.00
23.50
27.26
31.02
0.0025
136,000
PS No. 200
Straight-run
fuel oil
7.50
ft3
185.1
27.08
0.142
145.2
22.75
-
1.283
196.455
196.5
210.4
215.0
219.6
224.3
228.9
233. 5
252.0
270.5
289.1
335.3
381.6
0.072
Ib
14.11
3.144
0.0240
10.74
1.082
-
0.0621
15.0521
15.05
16.11
16.46
16.81
17.17
17.52
17.87
19.28
20.69
22.11
25.63
29.16
0.16
142,000
PS No. 300
Low-crack
fuel oil
8
ft3
179.1
27.61
0.130
140.5
19.18
0.011
1.242
188.673
188.7
202.1
206.6
211.1
215.6
220.0
224.5
242.4
260.3
278.3
323.0
367.8
0.069
Ib
13.66
3.207
0.0220
10.39
0.9118
0.0005
0.0601
14.5914
14.59
15.62
15.96
16.30
16.64
16.98
17.32
18.69
20.05
21.42
24.84
28.25
0.15
146. 000
PS No. 400
Heavy- crack
fuel oil
8.33
ft3
177.2
27.86
0.142
139.0
17.86
0.011
1.228
186.101
186.0
199.4
203.8
208.3
212.7
217.1
221.5
239.3
257.0
274.7
319.0
363.3
0.076
Ib
13.51
3.236
0.0240
10.28
0.8491
0.0005
0,0595
14.4491
14.45
15.46
15.80
16.14
16.48
16.81
17.15
18.50
19.85
21.21
24.58
27.96
0.17
152,000
a. Combustion products calculated for combustion with air 40% saturated at 60°F. All volumes measured as gases at 60°F. Moisture in
fuel included where indicated.
b. Maximum accuracy of calculations: 1:1,000.
Source: Air Pollution Engineering Manual, EPA AP-40, 2nd Ed., 1973.
-------
Burner Types
A great deal of ingenuity has gone into the design of commercial afterburners.
Experience shows that economic and performance advantages accrue to systems that
incorporate uniform and short flame zones, maximum contact between the fumes and
the flame, and intensive mixing. Beyond the initial combustion and mixing zone, the
design features are less critical, so that simple cylindrical or rectalinear sections
tend to be used. Cooling may have to be provided beyond the combustion zone to pro-
tect the blower and stack. This may be accomplished by heat recovery or by the in-
troduction of outside air.
There are two types of burner designs, based on arrangement: distributive and
discrete.
Natural gas and propane have commonly been used for afterburners, since gaseous
fuels are adaptable to uniform and short combustion zones. Distributive burners, shown
in Figure 5A, allow the use of the waste gas stream for combustion air and are compact
and efficient.
The line-type burner, shown in Figure 5B, uses a gas manifold or multiport con-
struction which injects gas into a network of divergent openings from a metal plate.
The waste gas enters through perforations in the sidewalls of the plate and is mixed
with the gas by the jet action, forming a short flame zone beyond the plate. Flame
contact is extended by this design. The metal plate must be highly temperature resis-
tant to avoid damage, and the amount of preheat is limited to approximately 1000°F for
the same reason.
A variant of the same general type, the multijet burner, is shown in Figure 6. It
differs in that a ceramic burner grid may be used. Gas and air enter the conical mix-
ing and combustion zone from the upstream face. This type of construction is less
subject to heat damage, but part of the waste gas stream must be bypassed if a stable
flame is to be obtained. Flame contact is lost with the portion of fume bypassed, and
the combustion chamber must be extended to allow for mixing of the gases beyond the
flame. Figure 6 shows baffles used to provide mixing.
Where oil is burned, or only interruptible gas is available, a single premix burner
is commonly used. The air supplied to the burner may be outside air, or if the waste
gas stream is reasonably clean it may be withdrawn from the ducting upstream. An
arrangement of this type is shown in Figure 7. Since flame contact in this type of sys-
tem is relatively poor, some arrangement is needed to obtain rapid mixing of the flame
with the waste gas. Baffles may be used for this purpose, or turning vanes in the air
inlet zone. Occasionally, a tangential inlet for the gas stream is provided in a cylin-
drical combustion chamber. The artificially induced swirl provides mixing throughout
the combustion zone.
Afterburners have been constructed in both horizontal and vertical configurations.
The horizontal type is more compatible with heat recovery systems, while the vertical
type (under forced draft) can reduce the cost of the stack.
68
-------
To Stack
Fume
X
r
Natural
Gas
A. Afterburner with Distributed Burner
BURNER
PROFILE
OPENING
PROFILE
PLATE
INCINERATOR
HOUSING
COMBUSTIFUME BURNER
B. Maxon Combustifume Burner
Source: Afterburner Systems Study, Shell Development Company, 1972.
Figure 5. Maxon Combustible Burner
69
-------
Natural
Gas
Natural
Gas
Fume
A. Hirt Multijet Gas Burner
\v\\\\\\v\ \\\\\\\\\\\\\\\\v\
Fume
B. Afterburner System Employing Multijet Burner
Source: Afterburner Systems Study, Shell Development Company, 1972.
Figure 6. Hirt Multijet Gas Burner
70
-------
Fuel-
Combustion
Air
(Fume
*• Exhaust
Source: Afterburner Systems Study, Shell Development Company, 1972
Figure 7. Afterburner Using a Discrete Burner
Accessories and Controls
Direct-flame incinerators will require thermocouple temperature sensors, used
to monitor:
• Inlet waste gas temperature. If the waste gas is preheated, a preheat bypass
control is sometimes used to prevent the temperature from going above safe
maximum inlet values.
• Temperature in the combustion area. Temperatures much in excess of the re-
quired minimum result in higher fuel and maintenance costs; for this reason,
the fuel injection rate is controlled through a burner control that incorporates
a sensof.
• Temperatures downstream from the combustion area, especially if heat re-
covery is a part of the system.
In addition, sensors and controls are frequently provided to monitor or control
pressure drops across the incinerator and combustion fires. The safeguards used
against combustion fires include flame detectors, automatic shutdown provision in
case of flame-out, and pressure switches. These devices are based on approval by
safety and fire protection organizations such as the Underwriters Laboratory, Inc.,
the Factory Mutual System, the Factory Insurance Association, and the National Fire
Protection Association.
71
-------
CATALYTIC INCINERATION
The catalytic incinerator differs from the direct-fired unit in that an active cata-
lyst is used to reduce the ignition temperature of the solvent in the waste gas stream.
Since less heating of the incoming gases is required, fuel consumption is reduced.
Further benefits include the smaller combustion chamber and the reduced equipment
maintenance associated with the lower operating temperatures.
Structurally, a catalytic incinerator differs from a direct-fired unit in that a
burner system in a preheat chamber is used to raise the temperature of the incoming
gases to 600°F - 900°F. The hot gases are then passed through a catalyst bed where
the fume is burned, releasing further heat and elevating the gas temperature to 800°F -
1100°F. Since no flame contact is involved, the preheat section is frequently a dis-
crete burner followed by a simple mixing zone, although a distributive burner may also
be used. Little or no combustion occurs beyond the burner, so the residence time at
peak temperature can be quite short. The net result is a somewhat smaller physical
system than the direct-fired unit. Figure 8 is a diagram of atypical catalytic incinerator.
Gas Conditioning
The catalysts used in catalytic incineration, normally platinum or palladium, are
extremely sensitive to contamination and catalyst failures are common. Heavy metals
such as mercury, arsenic, lead, and zinc will inactivate the catalytic surface. Plastic
resins and tar like materials may coat the catalyst, as may otherwise-inert materials
such as dusts and metallic oxides. As a result, the incoming waste gas stream must
be completely free of materials of this type. Where doubt exists as to the contaminants
in the gas stream, it is wise to use thermal incineration.
Where the advantages of catalytic incineration appear to warrant the expense of
precleaning gases, fabric filters or electrostatic precipitators may be used for dust
and scrubbers may be used for either particles or heavy liquid droplets. Water vapor
Fume Stream
70-400°F"
Preheat Catalyst
Burner Element
^^^^^~ 600-900°F
1
800-100°F
Clean Gas
to Stack
Combustion/Mixing
Chamber
Optional Heat
Recovery
(Regenerative or
Recycle System)
Source: Afterburner Systems Study, Shell Development Company, 1972.
Figure 8. Schematic of Catalytic Afterburner System
72
-------
will not affect the incinerator, although the temperature loss associated with water
scrubbing will increase fuel requirements.
Catalytic Combustion
The degree of solvent vapor oxidation that can be expected on a catalyst is affected
by the vapor composition, the reaction temperature, the surface area of the catalyst,
and the degree of contact between the solvent and catalyst. The last two variables will
depend on the commercial design of the catalyst and its support structures. A gener-
alized curve showing the relative effect of catalyst volume/flow ratio on combustion
efficiency is given in Figure 9. It is normal practice to follow the manufacturer's
recommendation concerning catalyst volume for a given application, since much of the
applications technology is based on field experience.
100 r
80
60
40
20
0.5 10 15 20
Volume of Catalyst/Volumetric Flow Rate of Waste Stream (Relative)
25
Source: Afterburner Systems Study, Shell Development Company, 1972.
Figure 9. Combustion Efficiency as a Function of Catalyst-Volume/Flow Ratio
73
-------
Both the solvent used and its concentration influence the design preheat tempera-
ture for the catalyst. Figure 10 shows catalyst temperatures required for combustion
of typical solvent hydrocarbons with some of the common combustible gases. These
temperatures must be modified to reflect the concentration of solvent. As may be seen
from Figure 11, lower vapor concentrations are considerably more difficult to oxidize
because of chemical kinetics and the lack of additional heat gain from the catalyst.
Commercial catalysts consist of small quantities of platinum or platinum/palladium
alloys deposited on metallic or ceramic support structures. One manufacturer electro-
plates the active metal onto the surface of fine nichrome ribbon that has been geometri-
cally packed to obtain a high surface area and void ratio. The nichrome structure is
quite inert and will withstand temperatures up to 1500°F without damage. Several
200
600
Temperature, °F
800
1000
1200
Source: Afterburner Systems Study, Shell Development Company, 1972.
Figure 10. Typical Temperature-Performance Curves for Various Molecular Species
Being Oxidized Over Pt/AI2O3 Catalysts
74
-------
tn
Solvent concentration
in waste stream
300
600 700
Preheat Temperature, °F
900
1000
Source: Afterburner Systems Study, Shell Development Company, 1972.
Figure 11. Effect of Solvent Concentration on Required Preheat Temperature
-------
other manufacturers use a ceramic base coated with a porous wash coat of aluminum
oxide. The catalyst is deposited on or in the porous surface coat in the form of small
crystals. The base structure may be pelletized or, more commonly, formed into a
honeycomb structure. The aluminum-oxide-based form provides somewhat greater
catalyst activity than the all-metal form.
Process Design Principles
For catalytic incinerator process design, it will be necessary to determine the
degree of preheat required for the solvent fume, emission concentration, and the ef-
ficiency desired. The subsequent heat and volume calculations are similar to those
for thermal incinerators, except that the heat is released in two stages. The cross-
sectional area of catalyst required is based on manufacturers' recommended face
velocities, which range from about 5 to 35 feet per second. A typical calculation of
the fuel requirements and temperature profile follows:
Example 5—Given: A waste gas stream of 4500 acfm at 200°F contains SOOppm of
hexane. The gas stream is to be catalytically incinerated with a
desired efficiency of 90%. Natural gas is available as a fuel.
Find: The preheat fuel required, and the temperature and rate of gas
flow at the exit of the catalyst.
1. Since natural gas is available, a distributive preheat burner will be used to
avoid the introduction of outside air.
2. From Figure 10, it may be seen that a catalyst temperature of approximately
900°F should be sufficient. Figure 11 confirms this temperature, at hexane
concentrations of less than 10% LEL. Since the temperature rise over the
catalyst will be small with only SOOppm of hexane, the conservative assump-
tion of 900°F preheat temperature will be used to size the burner. The natu-
ral gas flow will actually be slightly less than the calculated value.
3. Waste gas flow in scfm
„,-«« 460+60
= 45°° X 460T200
= 3545 scfm
4. Heat Input (neglecting contribution from hexane)
= (Available heat at 900°F, 0% excess air) x G
+ credit for initial heat of combustion air to 200°F
where G = scfm natural gas required
76
-------
• From Table III, available heat at 900°F
- 797.7 Btu/scf
• Credit for initial heat
3,11*
,
- G x 10.36 — - x enthalpy difference (200°- 60°)
SCI ^cLS
• From Table II, enthalpy difference (200°- 60°)
= 2.58 Btu/scf air
• Thus, heat input
- 797.7 xG +G x 10.36 x 2.58
= 824. 4 x G Btu/min
5. Heat Consumption
= (scfm waste gas - scfm needed for combustion) x
enthalpy change (900°F-200°F)
• From Table II, enthalpy difference (900°F -200°F)
= 2.58
• Thus, heat consumption
- (3545 - 10.36 x G) (15.92 - 2.58) (from Table II)
= 47290 - 138.2 xG
6. Heat Balance: Heat Input = Heat Consumption
• Thus, 824.4 G = 47290 - 138.2 G
• Solving for G , G = 49 . 1 scfm natural gas
7. The combustion gas at this point will consist of:
(i) air = 3545 - 10.36 x 49.1 = 3036 scfm
From Table III, products of natural gas combustion
(ii) CO2 - 1.134 scf/gas x 49.1 = 55.7 scfm
77
-------
(iii) H2 O = 2.083 scf/scf gas x 49.1 = 102.3 scfm
(iv) N2 - 8.236 scf/scf gas x 49.1 - 404.4 scfm
8. Heat available from hexane burning over catalyst
• Hexane flow rate = 3545 x —— = 1.064 scfm
• From Figure 4, at 900°F and 0% excess air, approximately 72% of gross
heat from hexane is available. Gross heat value for hexane is 4762 Btu/scf
(from Table I)
• Heat available from hexane
- 0.72 x 4762 x 1.064 Btu/min
= 3648 Btu/min
9. Combustion air required to burn hexane (from Table I)
= 45.26 scf air/scf hexane x 1. 064 scf
= 48.2 scfm
Products of hexane combustion (from Table I)
CO2 = 6.0 x 1.064 = 6.4 scfm
H2O = 7.0 x 1.064 = 7.5 scfm
N2 = 35.76 x 1.064 = 38.0 scfm
10. Combustion gases after the catalyst will consist of (from steps 7 and 9):
(i) air = 3036 - 48.2 = 2987.8 scfm
(ii) CO2 = 55.7 + 6.4 = 62.1 scfm
(iii) H2O = 102.3 +7.5 = 109.8 scfm
(iv) N2 = 404.4 + 38.0 = 442.4 scfm
11. Temperature at the end of the catalyst zone can be calculated by equating heat
available from hexane (step 8) to the heat consumed in temperature increase
in the combustion gases from step 10.
• Assuming a linear enthalpy change between 900°F and 1000°F for air, CO2,
H2O, and N2, the heat consumed by combustion bases (from Table II)
78
-------
2 B 0S°f x 2987.8 scfm air at AT
100 F
+ 3.22 e x 62.1 scfm CO2 x AT
+ 2.43 o x 109.8 scfmH20 x AT
+ 1.97 B 0SCf x 442.4 scfm N2 x AT
100 F
= 73. 14 x AT Btu/min
• Equating this to heat available from combustion of hexane (step 8)
73.14x AT- 3648
T - 50°F
• Thus the final gas temperature
= 900 + AT
- 950°F
12 . The final flow rate , from step 10
= 2987.8 + 62.1 + 109.8+442.4
= 3602 scfm
= 9767 acfm at 950° F
These are typical calculations for sizing a catalytic afterburner.
Accessories and Controls
The accessories and controls for a catalytic incinerator are similar to those re-
quired for a direct-fired unit and include a temperature-regulated preheat burner sys-
tem and a flame sensor for emergency shutdown. The burner control may be activated
by the post- catalyst temperature to smooth out fluctuations in final temperature due to
solvent variations .
Catalysts tend to become less effective with time, even with a clean waste-gas
stream. The normal life will vary from a few months to 2 or 3 years, depending on
the nature of the waste load. As the incinerator becomes less efficient, operating
temperatures must be increased to offset the decreasing catalyst activity. Frequent
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or continuous effluent monitoring for hydrocarbons or carbon monoxide is the only way
to be sure that emission regulations are being met unless the unit is operated at exces-
sive temperatures. However, continuous monitoring is expensive, as is overheating.
PROCESS BOILERS
The use of boilers—existing or planned—for fume combustion may enable significant
reductions in both capital and operating costs. The initial saving of the cost of a fume
incinerator will be somewhat offset by the cost of boiler modifications and ducting.
However, if the distance between the fume source and the boiler is not great, long-
term savings can be substantial. An important factor is circumventing the cost of
additional fuel for air pollution control only. In incineration, fuel is the major cost;
further, the lighter grades used (gas or distillate oil) may be difficult to obtain.
There are a number of conditions that must be met for a boiler to function satis-
factorily for fume incineration:
• The fume should be almost completely combustible. If not, the solids present
will either foul the heat exchange surfaces or cause the boiler emissions to ex-
ceed applicable particulate emission regulations. If there are significant quan-
tities of solids in the waste gas, the costs of increased maintenance of the
boiler and/or control of the particulates may well exceed the purchase price of
a fume incinerator.
• The contaminated gas stream should, preferably, constitute only a small frac-
tion of the air requirements of the boiler. If the volume of the gas stream is
large, special attention must be paid to the oxygen balance, mixing, and con-
tinuation of the air flow when the fume-emitting process is shut down.
• The oxygen concentration of the contaminated gas stream should be close to that
of air to avoid incomplete combustion, which can produce tars that coat heat
exchanger surfaces.
• The boiler must operate at all times when fume incineration is required.
• The fumes must be free of compounds, such as halogenated hydrocarbons, that
accelerate corrosion of the boiler.
In addition:
• Baffling may be required in the combustion chamber to ensure adequate mixing
and combustion of the fumes without bypassing.
• If the boiler-firing rate varies greatly, it may be worthwhile to install a small
auxiliary boiler that will operate under steady load conditions to produce a base
quantity of steam and serve as a fume incinerator.
Before a process boiler is used for fume incineration, a careful analysis should
be made of the operations involved. In a new facility, it is generally possible to plan
the layout and characteristics of individual boiler units to ensure that they will be
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economical and effective. In existing facilities some problems are inevitable, and an
objective analysis is necessary to make certain that projected economies are realistic.
VAPOR ADSORPTION
Although adsorption of organic vapors is a proven technique for their control, its
use in the metal coating industry for solvent vapor control is not common, for the fol-
lowing reasons:
• The low concentrations of solvent vapors in gas streams from metal coating
operations make this technique very expensive.
• Flow rates associated with metal coating are large and the large adsorption
units necessary for vapor removal are not cost-effective.
• Vapors from metal coating operations contain mixtures of solvents, and the
solvents used frequently change, depending on coating needs. This makes
solvent recovery by adsorbers impractical because of high costs of solvent
separation, unless the solvent mixture can be used elsewhere as a secondary
fuel.
• Process gas must be cooled to less than 100°F for adsorption, requiring addi-
tional cooling equipment.
• Adsorption beds are extremely susceptible to fouling from particulate matter
in the process gas. The effective life of the adsorption medium can thus be
prohibitively low.
• Under certain circumstances, low-temperature adsorption systems may re-
quire corrosion-resistant construction materials, increasing the initial outlay.
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CHAPTER II
HEAT RECOVERY
Since fume incineration involves heating significant flows of air to temperatures as
high as 1500°F, the potential for heat recovery is obvious. For large flows in thermal
incinerators, heat recovery is nearly always economically justified. Catalytic incin-
erators operating at lower temperatures offer less opportunity for heat recovery, as
do small incinerators where the increased capital requirement may exceed the value of
the heat recovered over a reasonable period of time.
Heat may be recovered and reused in a number of ways. The most common is pre-
heating the incoming contaminated gas stream to directly reduce the fuel needed for the
desired operating temperature in the incinerator (primary heat recovery). Where the
fume source is an oven, part of the incinerator exhaust gas may be used to heat the
oven, either directly or indirectly. Heat may also be recovered as either hot air or
steam for use in plant operations unrelated to the fume source (secondary heat
recovery).
The choice of whether to use heat recovery, and in what form, depends on a num-
ber of process and economic factors. These are summarized in Table V. In this
table, "effectiveness ratio" is the percentage of actual heat recovery relative to that
theoretically obtainable.
Primary heat recovery for reducing fuel required for incineration is almost uni-
versally accomplished by using either cross-flow tubular or regenerative heat ex-
changers. The tubular type provides relatively low-cost cross-flow configurations for
the low-pressure differentials encountered in incineration processes. Single- and
double-stage units are illustrated in Figure 12. Since the cost is almost directly pro-
portional to the number of stages, tubular exchangers are most commonly used in
single-stage applications, where 40-50 percent heat recovery is considered adequate.
The units are sensitive to fouling of the tube surfaces and are difficult to clean because
of the complex arrangement of tubes. They should, therefore, be avoided where soot,
tars, or possible polymerization products are present in the stack gases.
For high-efficiency heat recovery, regeneration by rotary heat exchangers is most
commonly used. An example of this type is shown in Figure 13. A wheel with large
surface area and sufficient bulk for a large heat capacity is rotated between the hot and
cold gas streams. Heat is captured by the portions of the wheel exposed to the hot gases
and lost as the wheel is rotated into the cool gas stream. Since the wheel is constantly
exposed to heating and cooling, warping and thermal stress are potential problems in
metal wheels and sophisticated construction is needed to overcome them. In recent
years, however, ceramic materials with low thermal expansion (and freedom from
corrosion) have been successfully used for the wheels.
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Table V
Comparison of Heat Recovery Techniques
Type
Tubular
exchanger
gas/gas
Regenerative
(rotary)
exchanger
Effectiveness
Ratio, E
1 sta-50% max
2 sta-62% max
3 sta-85% max
up to 85%
Additional
Auxiliary
Equipment
Limitations and
Problems
1. May be easily
fouled; frequent
cleaning and
maintenance.
2. Failures, differ-
ential thermal
expansion.
3. Hot surfaces may
crack or polymerize
fume components,
lay combustible
deposit, initiate a
fire.
4. Bulky, heavy, added
roof load and/or
floor space.
5. Corrosion if cools
below dew point of
flue gas.
1. Easily fouled. Use
only on relatively
clean streams.
2 . Burnout if failure on
rotary drive motor.
3. Requires attention
to pressure balance
to control leakage
at seals.
4. Avoid cooling flue
gas to dew point,
but otherwise is
relatively insensi-
tive to corrosion.
5. Ignition if overheat
fuel-rich stream .
Common Use
Primary,
secondary
heat recovery
Secondary heat
recovery, to
heat air enter-
ing oven
83
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Table V (continued)
Type
Flue gas
recycle
to oven
Steam,
generators ,
boilers,
water
heaters
Heat pipe
Effectiveness
Ratio, E
30%
to 75%
to 60%
Additional
Auxiliary
Equipment
Safety
Controls
Extra burn-
ers and con-
trols; safety
controls
Extra duct-
ing, blowers
controls
Limitations and
Problems
1. Process must be
compatible with flue
gas (condensation?
sulfur in fuel? CO
or CO 2? reduced
oxygen ? unburned
fuel?)
2. Usefulness depends
on temperature and
heat requirements
of fume generating
process.
1. Ties steam genera-
tion to fume process
and vice versa.
2 . Match steam heating
load to afterburning
heat release.
3. Dew point and con-
densation on cold-
water coils.
1 . Can only be used for
hot side tempera-
ture up to 800 °F
Common Use
Secondary heat
recovery
To preheat oven
air, secondary
heat recovery
Adapted from: Afterburner Systems Study, Shell Development Company, 1972.
Rotary heat exchangers are capable of very high efficiency arid offer significant
cost and space savings over other high-performance heat exchangers. Fouling can be
a problem if stick materials are handled, but inert dusts are tolerated at reasonable
concentration. Cleaning is easier than with tubular-type exchangers.
The principal operating problem with rotary exchangers has been leakage from the
seals separating the hot and cold gas streams. Since these seals must tolerate motion
while subjected to temperature extremes, finding a substance and a design that would
give reasonable life expectancy was technically difficult. However, in recent years
manufacturers have managed to overcome this problem, at least for relatively clean
gas streams.
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Fume
Stream
A. Shell and Tube Type Exchanger Cross-Flow Type
Cooled
Flue
Gas
Cold
Fume
Stream
Heated
Fume
Stream
B. 2-Pass, Cross-Flow Exchangers (Arranged to Place Units Counter-Flow)
Source: Afterburner Systems Study, Shell Development Company, 1972.
Figure 12. Typical Shell and Tube Heat Exchangers
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Cool
Fume
Stream
Rotating
Matrix
Heated
Fume
Gas
Source: Afterburner Systems Study, Shell Development Company, 1972.
Figure 13. Rotary Regenerative Heat Exchanger
A heat recovery technique known as heat pipe is also commonly used. Here a re-
frigerant inside a series of tubes circulates between the hot and cold sides (Figure 14).
As the liquid refrigerant enters the hot side of a tube, it absorbs heat from the hot
gases and evaporates. The evaporated refrigerant then gives up its heat to the cold
air stream and condenses. In this way, heat can be transferred from hot gases to cold
air. Heat pipes are used for secondary heat recovery purposes like comfort heating.
A limitation of the heat pipe technique is that the maximum hot-side temperature can-
not be much in excess of 800°F. For this reason, the technique is used in conjunction
with ovens on coating lines that have water-based or powder-deposition paint technolo-
gies, where incineration of the oven exhaust is not required.
In cases where additional process steam or heat is required in a plant, it may be
more economical to let the fume incinerator serve the dual function of controlling air
86
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hot side
cold side
hot gases
evaporation
condensation
tube filled
with refrig-
erant
cold air
Figure 14. Heat Pipe
pollution and providing process heat. If separate boilers are constructed for process
heat and for heat recovery to reduce fuel consumption in the incinerator, the total cost
will be higher than for a single burner and heat recovery unit. For true economy,
plant operations must be such that incineration of waste gases would not be necessary
at times when the heat recovered from them could not be used in processes.
The process heat or steam available from afterburners of even moderate capacity
can be substantial, as is shown in Figure 15. Recovery may be accomplished by burn-
ing the contaminated gas stream in a boiler, by using an afterburner followed by only
the heat exchange portion of a boiler, or by using a conventional gas-liquid heat ex-
changer to produce a hot-fluid stream for process use.
Where the fume source is an oven, it is common to circulate part of the exhaust
gases from the afterburner back to the oven to provide either part or all of the heat
requirement. Care must be taken to prevent the exhaust gases from harming the prod-
uct being processed in the oven. Temperature control may require sophisticated ex-
haust and outside air blending, but safety is enhanced by a low oxygen atmosphere in
the oven. The oxygen needs for fuel combustion prevent use of a totally closed system
and result in more exhaust gas than can be used in the oven. Thus, the total heat re-
covery efficiency is low. However, exhaust-gas recirculation can be used with heat
recovery for other purposes to obtain a high overall recovery efficiency.
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Flue Gas Temperature
Note:
Final Temperature of Flue Gas = 350°F
Fume Stream Used for Oxygen Supply
(No Outside Air Used in Combustion)
4.000
8,000
Afterburner Capacity, scfm
12,000
16,000
Source: Afterburner System Study, Shell Development Company, 1972.
Figure 15. Process Heat Recoverable from Afterburner
88
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The following calculation shows the effect of primary heat recovery on afterburner
fuel requirements in a sample case:
Example 6— Calculate the fuel savings expected by the application of a heat exchanger
of 50% effectiveness, as shown in the schematic:
1400°F O: Oven
A : Afterburner
P: Primary Heat Exchanger
using data described in Example 2 (oven exhaust = 3000 acfm at SOOT, containing
500ppm toluene).
1. Heat exchange effectiveness
0
300° F
i
P
A
where: C = specif ic heat of gas , Btu/°F
TIO = Oven exhaust temperature entering the heat exchanger, °F
Too - Oven exhaust temperature leaving the heat exchanger, °F
T. = Afterburner exhaust temperature entering the heat exchanger, °F
1 A
TQA = Afterburner exhaust temperature leaving the heat exchanger, °F
To avoid using C values between unknown temperatures, the above expression can be
converted in terms of enthalpies, H:
Hoo~Hio
e = -
HIA ~Hio
Where subscripts have the same meaning as above.
• In this example, e = 0.50
2. Oven exhaust temperature leaving the heat exchanger, TQO
• Assuming all gases are air, from Table II
H-4.42
e =
26.13-4.42
Hnr. - 15.28 Btu/scf
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• From Table II, for air, the corresponding temperature
Too « 868° F (by linear interpolation)
3. Heat input, as in Example 1 and 2
= (Available heat at 1400°F at 0% excess air) x G
+ (Credit for preheat of combustion air from 60° F to 868 °F)
+ (Available heat from toluene)
S of 3.1T*
- 668 G + 15.28 — - x 10.36 — ; - x G
scf scf gas
+ 0.61 x 4605 + 44.04 x 15.28
= 826 G + 3482 Btu/min
4. Heat consumption at the afterburner
- (2053 - 10.36xG - 44.04) x (26.13 - 15.28)
= 21798 - 112.4 x G Btu/min
5 . Heat Balance : Heat Input = Heat Consumption
826G + 3482 = 21798 - 112.4 x G
• Solving for G ,
G = 19.5 scfm natural gas
A reduction in fuel consumption from 43.3 scfm (Example 2) to 19.5 scfm is achieved
by the introduction of a heat exchanger of 50% effectiveness.
The final afterburner exhaust temperature (leaving the heat exchanger), TQA, can
be calculated as follows:
6. Heat Balance: Heat absorbed by oven gases in the heat exchanger - heat lost
by afterburner exhaust, or
TT TT __ TT TT
nOO IO IA OA
15.28 - 4.42 - 26.13 - HQA
HQA = 15.28
90
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• From Table n, for air, the corresponding temperature
TQA« 868°F
The heat contained in the incinerator exhaust at 868°F can be utilized for oven heating
as well as to meet a portion of total plant steam needs. If the incinerator exhaust is
used for steam production in a waste heat boiler, and if we assume the final incinerator
exhaust temperature at 350°F, the heat available for steam production can be calculated
as follows:
7. Heat available for steam production
= 2053 scfm x (Hg68 -H35Q )
= 2053 x (15.28 - 5.36) from Table II
= 20,366 Btu/min
- 1.22x 106 Btu/hr
This would be roughly equivalent to 1, OOOlbs/hr of low pressure steam.
The choice of a heat recovery system should be made after analyzing the inciner-
ator heat capacities and the total heat needs of the plant, using the steps below as a
guide:
1. Determine the heat recoverable from the incinerator exhaust. This heat can
be used for preheating the oven or for generating steam, heating water, or
comfort heating.
2. Determine the preheat that can be applied to the oven exhaust, that is, the
highest amount compatible with the safe maximum oven exhaust temperature.
3. Determine the heat that can be recycled to the oven as incinerator exhaust by
using assumed exhaust to outside-air ratios and the known oven temperature
requirement. In considering this as an option, the effect of incinerator ex-
haust on product quality must also be evaluated.
4. Determine the purposes for which heat recovered from the incinerator exhaust
could immediately be used. During periods of incinerator operation when there
is no hot water or steam demand in the plant, the value of heat recovery is
limited. Heat recovery for comfort heating can be used only in the winter
months.
5. Determine the heat load from steam, hot water, and comfort heating, based
on past history of fuel burned for these purposes.
If the total heat demand from steps 2,3, and 5 is comparable to the amount of heat
recoverable, then the plant should consider heat recovery units with high effectiveness
ratios (Table V). If the heat demand is small compared to the recoverable heat, heat
exchangers with lower effectiveness can be used, thereby reducing capital costs.
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CHAPTER III
COST OF COMBUSTION AND
HEAT RECOVERY SYSTEMS
The combustion of dilute organic fumes in air streams is one of the most expensive
forms of air pollution control. In smaller units, installed costs may range up to $25
per cfm capacity. Operating costs are also high because of the fuel consumed in heat-
ing air streams to high temperatures. Heat recovery will reduce fuel consumption but
entails higher capital investment. Selecting a combustion and heat recovery system
for a specific use requires careful consideration of capital and operating-cost tradeoffs
to insure a minimum overall cost.
Installation costs for thermal incinerators range from 75 to 200 percent of the cost
of the basic equipment, averaging about the same as equipment costs. Installation costs
will normally consist of installation labor and materials, plus auxiliary equipment that
consists of:
• Ducting;
• Blower motor controls and instrumentation;
• Insulation;
c Blower motor housing for noise control;
• Instrumentation, including air pressure regulator, temperature monitoring and
recording, flame safety controls, and fuel rate monitoring; and
• Foundations and structural steel.
The installed costs of thermal and catalytic incinerators, with and without heat ex-
changers, are shown in Figure 16. Estimates were based on roof-top location with al-
lowance for structural steel and assuming custom-designed units. The cost, therefore,
may be considered as in the upper limits in the less-than-10, 000 cfm range. Pre-
engineered units with a few thousand cfm capacity may be installed at a total cost of
approximately two-thirds that of a custom-designed and fabricated unit.
Costs of operation will depend on the number of shifts, the temperature of the con-
taminated air stream, and the incineration temperature. Variable cost factors are
fuel and electricity consumed and labor for operation. Maintenance, taxes, insurance,
and overhead charges on space are commonly taken as a fraction of capital costs. Fig-
ure 17 shows estimated operating costs for various incinerator configurations. All
costs are based on an 8-hour single shift operation, inlet fume temperature of 70°F,
electricity at $0.03/kWh, direct operating labor of 0.5 hr/shift, and miscellaneous
92
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275
250
A. Thermal without heat exchanger
B. Thermal with primary heat exchanger
C. Thermal with primary and secondary heat exchanger
D. Catalytic without heat exchanger
E. Catalytic with primary heat exchanger
225
200
175
a 150
125
100
75
50
10
15
Flow, scfm x 1Q3
20
25
30
Figure 16. Capital Cost of Incineration
93
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245
220
A. Thermal without heat exchanger
B. Thermal with primary heat exchanger
C. Thermal with primary and secondary heat exchanger
D. Catalytic without heat exchanger
E. Catalytic with primary heat exchanger
195
170
145
120
95
70
45
20
10
15
Flow, scfm x
20
25
30
Figure 17. Annual Variable Cost of Incineration
94
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costs of 9 percent of capital. The operating cost would increase by slightly less than
factors of 2 or 3 for two- or three-shift operations.
The total annual cost of ownership may be derived from Figures 16 and 17 by com-
bining a suitable fraction of the capital cost with the annual operating cost. At current
interest levels, the annual cost of capital is commonly taken as 14-18 percent of total
investment. The data in Figures 16 and 17 have been left uncombined to facilitate the
approximation of total annual costs for multishift operations.
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SUMMARY
The first part of this publication was concerned with reducing hydrocarbon emis-
sions at the source through changes in coating formulas and plant equipment and
processes.
In this part we have discussed, in some detail, the end-of-line treatment of emis-
sions that must be dealt with after all other practicable control measures have been
taken.
Throughout the entire volume the emphasis has been on presenting pollution con-
trol techniques as sets of options, with enough information on each method—pro and
con, descriptions, diagrams, and simple calculations—for assessing its suitability for
a given plant.
In both sections of this publication, plant managers, engineers, and operating per-
sonnel should find the basic information they will need to plan, in logical sequence,
for reduction and treatment of hydrocarbon emissions from metal coating processes.
y.$.
*t*on *, Utoary (Pt-U»
77 Wtst Jackson Boulevard* 12th Fto»
CMeagML 60604-3590
96 *us GOVERNMENT P*IN™GOFFICE 1977-757-056/6563
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ENVIRONMENTAL RESEARCH INFORMATION CENTER • TECHNOLOGY TRANSFER
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